I've had OZ at JFK ask me for my onward ticket flying JFK-ICN-BKK. I've had EVA ask me for an onward ticket flying JFK-TPE-NRT. I've had the NH people at the JFK BA lounge ask to double check my Australian e-visa (they basically asked if I had one) flying JFK-NRT-BKK-PER.
I didn't take it personally, I had onward flights so it wasn't an issue.

If you got to Thailand you could have been asked for an onward ticket. If you didn't have one and immigration didn't like your story and denied you entry...would the onus be on TG to ship you back off and pay the "denied entry" fine for a pax they brought there?