Because it's the only way to join a game in progress?
Blocking a user instead of just denying the hot-join would work for trolls like the one you're describing, but seriously: just add a password if your game isn't open to the public. I for example, might try to join any non-password protected game that appears in the lobby without even checking the name of the game as soon as it appears in the list cause I don't care where I end up and just wanna play for a bit with some random species or another in some random universe or another. (aka the typical random hot-joiner who may as well just be a 'smart AI' for all the difference you'll notice from my presence xD)
When I get bored enuff anywho..