Thousands of visitors from Asean countries fly into Thailand everyday for leisure, travel and business and I've never heard of anyone being asked to show a ticket out of Thailand upon entry although having a return ticket may be an airline requirement at the point of departure. In 20+ years of travelling to Thailand I've never been asked to show a flight ticket out of the country upon arrival. There may have been exceptions if the immigration officer in attendance suspected the visitor may have been abusing the social-exemption pass for reasons other than genuine travel and short-term business. And this applies at land borders too.