Or maybe you do force people to join, because that's the entire premise and function of the channel, you goddamn moron.
You are not forced to join if you (or rather someone with the same IP) has not been in there before. Also, every 45-90 minutes there is a 1 minute window in which you can freely part without getting forcejoined again.

A combination of IRC client programmers being dumbasses with no concept of security and Rizon staff being such chucklefucks that they take advantage of it to harass users.
This really has nothing to do with clients, IRCds have a command to forcejoin a client to a channel.