These servers are actually made to maintain themselfs.
Normally when people donate they get nothing i return.
On TF2 donating to certain servers gets you something.
For example:
1. You won't get auto-kicked for being AFK.
2. You can play any class, since servers have limits to some classes like engeneers.
3. My personal favorite, you can get a place reserved in the server, meaning no waiting to get in.
So the low cost of the servers+ tne get something when you donate feature makes it quite easy to mantain a server.