There are people on this forum who have been asking why the international airlines are hasseling travelers who try to enter Thailand without a visa. Here's some background information for those people.
The International Air Transport Association (IATA) is an international industry trade group of airlines headquartered in Montreal, Quebec, Canada. The executive offices are at the Geneva Airport in Switzerland
IATA maintains the Timatic database containing cross border passenger documentation requirements. It is used by airlines to determine whether a passenger can be carried, as well as by airlines and travel agents to provide this information to travelers at the time of booking.
This database is the successor to the Official Airlines Guide (OAG) which was a 3 volume list of rules and regulations that IATA member airlines must follow.
In that OAG, and now in the database, there is an Appendix that lists all visa regulations for entry into most countries and all IATA member airlines to follow for international travellers.
In the Thailand section of that appendix, it lists the countries whose nationals are allowed that 30 day visa exempt entry into Thailand.
In a footnote it also states to the airlines that the visa exempt entry can only be given if "the visa free passnger has an out-of-country (onward travel) ticket from Thailand". Note that it does NOT say a return ticket is required...it only wants an out-of-country ticket.
From that requirement, the international airlines as members of IATA derive their requirement that you either must have:
1. A valid Thai visa OR
2. An out-of-country ticket if you qualify for a visa exempt entry.
So for you who want to know why you need to purchase a out-of-country ticket to enter Thailand without a visa...that is the reason.