On a side note, without regard to the OP's specific situation, Delta's responsibilities, or Thai immigration law........ In the real world of Thai immigration, even if you (citizen of wealthy western nation) did arrive at BKK with no visa and no proof of onward travel there is a 99.9% chance they would not even ask for it. In close to 100 arrivals at BKK and a variety of Thai land borders I have been asked for proof of onward travel exactly once and that was during a short lived crack down on foreigners making visa runs a couple of years ago and even then, only because I have vast numbers of Thai immigration stamps in my passport. I haven't been asked since and that particular crack down ended after a couple of months. However even if you were the unlucky 0.1% and they DID ask and you did not have proof, there is a 99% chance you could just talk your way past it if you don't look like a bum and had a plausible explanation as to how you were not trying to overstay in Thailand illegally. And if even that failed, the ABSOLUTE worse case scenario would be they would turn you around and send you to the Thai Airways ticket counter to purchase a fully refundable, full fare ticket to 'someplace' which you could then turn around get your money back for once you got past immigration. No way are the Thais deporting any citizen of a rich western nation solely for lack of an onward ticket unless they are to destitute to front the money for a few days on an onward ticket or there was something else more serious wrong with them.