Most airlines will require proof of return if you are on tourist visa, but not round trip ticket.
Over last few decades I've been asked for the proof twice.
I had one way ticket Washington - London - Bangkok and the gate agent at London asked me for proof of flight out of BKK.
Same thing when I was flying Phuket - Singapore. The check in agent at Phuket airport asked me for proof of departure from Singapore.
Both times I showed them itinerary flying with different airlines and everything was good.
The airlines are trying to protect themselves. If the passenger flying on one way ticket on a tourist visa is denied entry, the airline get penalized (I think $3500 per person) + they have to cover that persons' flight back to the place where he/she came from before being denied entry to US in this case.
Heck, I'm flying Turkish Airlines one way next month. However, I'm flying on tourist visa and I have separate one way ticket for flight back with different airlines.