I don't think modding has anything to do with it beyond being the "in" place to be for the "show".

No servers are public or owned by IG... all servers are paid for by players. If someone wants to charge to be on their server it seems to be their right since they are paying for it.

If they can create an environment that attracts people for whatever reason then why not?

If it was just the game then why would people pay more for a certain place? So it isn't really the game itself they are paying there to be with.

I wouldn't personally and wonder why people do, but if they want to it's no skin off my nose.