It also hurts the game's image if many servers spam players with "Donate $20/mo for the MASTER rank and get exclusive weapons that deal twice the damage of normal weapons!"

I haven't played in a while but I remember playing Minecraft when I was still in school and servers were filled with this- many would let you donate hundreds for admin/OP/whatever, many would restrict core parts of the game to "donating" members (such as mining certain ores, going into certain areas, etc). Eventually it got so bad that Mojang actually had to step in and (vaguely) threaten to sue any server owner who does this - http://andrewtian.com/mojang-threatens-lawyers-against-pay-t...

Also, admins that can ban players for any reason, who are picked by the server owners (and not the devs), can result in backlash if someone influential is banned.