He fosters and encourages that behaviour.
I once saw a little interview with him where it was asked about the behaviour of some of his rabid fans on social media in regards to their treatment of other people on twitter who may express any form of criticism towards him.
His reply?......"Well they shouldn't mess with me".
Yeah, and in another interview his reply was, that he kind of likes it how "passionate" his fans can get. This was at the time he just started dating Selena, and she was receiving death threats.