It's not fundamentally different from paying for movies or TV or internet access when used for entertainment. Or buying novels to read. Or watching a play at a theater, and people have been paying for that for a looooong time. Or paying to get into a stadium to watch a sport or competition. Or paying to play laser tag. It's just, we've been socialized not to think of RPGs as a paid thing, and socialized to expect to pay for those counterexamples.

How do you react to the idea of paying for radio? That's pretty weird, right? But people do pay for satellite radio, even when free radio is available. Same thing with GMing.

Given this cultural standard, it'd be very hard to make a living as a full-time GM.