But Immigration rules are separated with stupid airlines' policies. You need to deal directly with the airlines if they refuse you to board or make new "rules" out of the blue. I have flown to Thailand with Air Premia, Air Asia, Thai Airways, T'Way. Never been asked about return tickets. The only weird time was once when the airline clerck was obssesed with asking me when would I return to my country (the origin country). I told her: whenever I want, that's not your busines haha. (She was not asking when was I leaving Thailand, rather when will I go back to my country. Said so, I can go anytime I want hahahaha, or even never again. Who cares?).