They could, if they didn't like the conditions, just migrate away and start their own phpbb board out of their own pockets. Web communities have historically been very good at policing themselves and segregation keeps the fringes at the fringe.

Reddit are merely their current landlords who let them live there rent-and-maintenance free.

I'm not supporting the actions of Reddit at all. But the internet is still the internet. If you don't like the way somebody does things, you can always just build your own alternative. Some server space and a URL is actually quite cheap.