Supposedly, it’s his wife’s family that has the $$$$. As a Hollywood producer, he was used to flying private, but as a public servant, he has to follow rules. The Fed does allow other-than-coach options, but you need a decent excuse. That’s not a law, but government rules, which these assholes violate as often as they breathe. Even DeVos has suposedly been flying to Florida to work, then jump on her yacht.
Another question, why does tRump travel with his cabinet secretaries? They aren’t his entourage, they’re supposed to be working.