There's lots of bans because lots of players misbehave, not going to go into further detail because everyone here has already explained it.
Don't share your accounts and don't behave in ways you shouldn't (unless you want to get banned)
If your friend feels like he did not deserve this then support.toonslab.com is a few seconds away.
http://www.reactiongifs.com/r/2013/05/h7C81F493.gif (Unsupported image host)