@Daniel *********
True, but for a DTV I'd seriously question anyone trying to get me to book an onward ticket. I'd try to talk my way out of it and tell the staff they're untrained and don't know what they're talking about.
I think the chances of them requesting an onward ticket on a DTV is extremely remote. The only staff who might do so, are those who automatically ask to see onward tickets for foreigners (non-Thais) flying to Thailand on any visa, because they just read the screen they see and assume all non-Thais need an onward ticket.
You can always ask for a supervisor in such cases and they'll override the decision made by the check-in agent.