the purpose of shops on servers is to tempt players with the promise that if they stay on the server long enough, eventually they too will get something powerful and get to terrorize all the people who can't afford whatever weapon they're using
it's basically being given fake assurance that you're doing well in exchange for you sticking around
it's a really gimmicky and underhanded way to make sure people are addicted to your server, and thus keep them around long enough to promote more people joining the server. it's also really stuff game design because you're giving one group of people that are supposed to be on roughly equal ground with everyone else way more powerful weapons on purpose
that's fine for a little while (see: ubercharge, powerups in other games) but the fact that you can then keep your power-up means that you can just lord that stuff over everyone else until they either get bored and leave or stick around and propagate the cycle
and besides, not nearly that many servers require cooperation (and in fact most servers actually are competitive), not every server is on the ball with glitches, and being able to escape the map does mean the server sucks because people outside the map can harass the players inside in ways the map maker didn't intend
