It is none of the airline’s business. That is an issue for immigration.

You may continue to think that, until, g-d forbid you have an encounter with an airline that thinks different than you do.
And then, as they say, Buena Suerte...

Please take it from me as someone who has experienced what can happen in this situation...

Came down yesterday on United from through Chicago. Never got asked anything about it there or here. I bought a return ticket (with miles) for late April on a different reservation. So, I might learn something on my return to the US in April, if a return ticket to Mexico is required. Ya think?

What country's passport you use is likely a factor. If flying to the US on as US passport you would obviously have NO requirement for a "return" ticket. If on a Mexican or other foreign country's passport, quite likely. My understanding is that it is difficult to get a visa for the US from many foreign countries.

Aren't they just ensuring you don't just illegally hang out in your travel destination like a hobo?

The reality is we need a Subject Matter Expert. We are just speculating. But I do believe you are correct regarding the policies of some countries. However, in my humble opinion, it is not the airline's job to enforce this unless duly authorized by the country's immigration officers. We are fairly certain that Mexico has no such policy so the Delta agents are either woefully misinformed or just blowing smoke and seeing if the hapless sucker goes along with their toothless edict.

Here's a delightful story of one such encounter with a country that does enforce "onward ticket" rules.

Article enumerating which countries have "onward ticket" rules.

Some of you are 'speculating' about this situation.
So why don't you at least consider the experiences of some travelers who have actually run into this type of situation.
I have at least twice and know of several friends who have also run into it.
Now admittedly this has never happened flying INTO Mexico, so perhaps you might want to disregard what I am about to say.
But my experience traveling has always indicated it is better to be informed of what might turn out to be an 'extreme' example.
The reason the airlines sometimes enforce this onward ticket policy is because if immigration in the country where the passenger lands denies entry, then it is the airlines obligation to return the passenger, at the airlines expense, to the country where flight originated or perhaps passengers home country. I'm not sure which...but I do know the airlines are required to 'move' the passenger elsewhere. And the airlines don't want to assume the financial responsibility of moving that passenger.
I may not have the precise details down because this exact situation has never happened to me.
But last year I was flying from Mexico City to Cartagena, Colombia and I was denied boarding because I had only a one-way ticket. I had to scramble and in 5 minutes I had to book a flight on the spot from Colombia to Florida. And by the way Colombia also has a requirement to fill out an immigration form ('Check-Mig') usually before boarding a flight, which the airline (Avianca if I remember correctly) required in this case.

As I say you may disregard this explanation because it may have nothing to do with flying into Zihuatanejo or Mexico City.
But remember the period a couple of years ago when travelers into Mexico were finding they had been stamped in for 7 or 14 days when they assumed they would get the traditional 180 days. And this was not very publicized beforehand.

Really, is an airline in a position to deny boarding to a person who says, “I’m flying to Mexico and driving back with a friend.”?

And if an airline were to try to impose such a requirement, the time to advise the passenger woukd be at the time of selling the ticket. “I’m sorry, but we cannot sell you a one-way ticket.”

Now, who are you going to believe? A friend, or some anonymous stranger on the internet?

Aren't they just ensuring you don't just illegally hang out in your travel destination like a hobo?

The reality is we need a Subject Matter Expert. We are just speculating. But I do believe you are correct regarding the policies of some countries. However, in my humble opinion, it is not the airline's job to enforce this unless duly authorized by the country's immigration officers. We are fairly certain that Mexico has no such policy so the Delta agents are either woefully misinformed or just blowing smoke and seeing if the hapless sucker goes along with their toothless edict.

Here's a delightful story of one such encounter with a country that does enforce "onward ticket" rules.

Article enumerating which countries have "onward ticket" rules.

A boarding agent wouldn't let us on a Delta flight in Atlanta without a return. We showed our Residente Permanente cards and it worked.

Pinche Delta! Why do they believe they have this type of authority over the right of travel? Did someone appoint them Mexican immigration officers? Or is it just greed that is motivating them? They simply want you to buy the return ticket with them? Either way, it is outrageous.

End of rant.

Coming down Saturday On a 1 way ticket. Didn't make a return reservation due to an Easter "glitch", so return is a little bit up in the air. But planning mid to late April. A friend advised me that a round trip ticket is required before an airline will allow you to travel. Correct or not?

A boarding agent wouldn't let us on a Delta flight in Atlanta without a return. We showed our Residente Permanente cards and it worked.

Coming down Saturday On a 1 way ticket. Didn't make a return reservation due to an Easter "glitch", so return is a little bit up in the air. But planning mid to late April. A friend advised me that a round trip ticket is required before an airline will allow you to travel. Correct or not?

Incorrect. At least for México. I have heard of other countries that require a round-trip ticket such as Fiji but you would have to verify this information. I routinely bomb down to Mexico with one way tickets.

In my experience it's not so much the country that may require...it's the airline.
it's definitely a consideration when I fly to Thailand.
It's never been an issue when I have flown into Mexico on a one-way ticket.
But you never know when you might run into the check in agent who may or may not know the 'rules'.
it's good to know about:
onward ticket.com

Coming down Saturday On a 1 way ticket. Didn't make a return reservation due to an Easter "glitch", so return is a little bit up in the air. But planning mid to late April. A friend advised me that a round trip ticket is required before an airline will allow you to travel. Correct or not?

Incorrect. At least for México. I have heard of other countries that require a round-trip ticket such as Fiji but you would have to verify this information. I routinely bomb down to Mexico with one way tickets.