Don't blindly trust Reddit. The unlukiest people on earth share their experiences here, often they are extremely rare cases. Sometimes trolls post fake stories or they didn't write all the truth.
I would add that Onward ticket is very fast to buy, a normal ticket require more time so this MAY raise suspicions...probably the guy was already suspicious because i did go so many times in Thailand and IO never asked me a return ticket.
there are some trolls with too much free time...immigration officers never asked me return tickets even when i didn't have any visa, and i've been many times in Thailand
Partecipante anonimo 528 is about being honest. You have to be a tax resident for an area. For me paying in Thailand or others countries is the same. If somebody don't like that, he can go in Dubai or Paraguay as a tax resident and pay nothing.