I voted rude.
I look at it this way: I com from a pretty big family. It is incredibly rare that I get to be the center of attention, for various reasons. If someone decided that they were entitled to crash in on the one time that I get to be in the spotlight, and took that attention away from me - and I know this sounds childish, but it's how I feel - I'd be more than incredibly upset. I'd be hopping mad. I would be angry and resentful.
It's great that everything turned out okay in this situation - but that doesn't make what he did right. The ends do not, in any way, justify the means.
I find it beyond rude that Justin Bieber - a young man that already has the attention of millions - feels entitled to take the attention away from a couple of random strangers, just because he can.
Just because you can, doesn't mean you should.