It's not legal trouble. Businesses do it all the time.
https://www.techdirt.com/articles/20130417/08024722740/ea-shuts-down-social-media-games-without-refunding-money.shtml
If HellsGamers has it in their ToC that anything you buy with real life money is more "this is ours, but you can play with it" rather than "this is now your property, do as you want with it", then yes, they can legally close down the server and see no legal repercussion.
http://www.gamerlaw.co.uk/2012/what-happens-when-a-game-with-virtual-goods-closes/
Secondly, it's just not realistic that someone would go through all the paperwork, money lawyers, and hassle to get back $20 that they paid. It's not worth it. If HG wants people on their server, this is what they have to do. Instead of bashing ideas, how about we throw new ones in there?