Again though, what is so hard about just asking before joining a raid? I guess I haven't heard of anyone just going in the vault of glass and hoping that people join his raid group from his friends list uninvited.
And what happens is that we are all on a party chat and not in-game chat, so we either have to invite the guy to the chat group just to explain to him why we are kicking him, or kick him without explaining, which is a bit rude.
It's not that I hope they never join. I want to play with them. People hop into my nightfalls, weeklies and heroic dailies all the time. It's just common courtesy to ask before joining someone else's raid which is almost always pre-formed.
Tbh, this behavior has gotten far, far worse/more common since the cheese tactics started. I think people are just joining for the checkpoint/easy cheese.