Because you don't know the truth. Fact is, if they are more concerned about the job, they'll show up with normal color hair and pearcings out. You won't even give them the chance. And quite frankly, the "weird stuff" they are into outside of work really isn't your business. I realize you may want to use that in your decision, and that's fine, but again, precluding someone from the interview who, for all intents and purposes appears to be a decent fit for the job, based entirely on ignorant stereotypes that have been debunked time and again is just plain bad management.
would you consider celebration of drug use to be disqualifying?
Like it or not, with the advent of social media, it has become our business, look at the example of people who have been fired for Tweets, because they do reflect on the employer.