Regarding conversation of foreign currency to USD and then to home currency (THB), this is how the majority of cards work, and not just in Thailand.
Card companies do this to manage risk and standardise procedures. They don't want to have to directly convert two obscure currencies because their wouldn't be much of a market for it and the rates would be horrific.
If you hired a car in Kazakhstan, they might struggle to exchange Tenge directly to Baht.
Does this rule benefit them with major currencies such as Euro, Pound, Yen, etc? Of course, they wouldn't have it any other way.