Because he forged a Certificate of Live Birth and put it up on his website and sent it out to the media as legit.
The forgery put out as legit is indeed a problem. Not just a legal problem but a known lie by a presidential candidate never looks good. Um, that is, if people hear about it.
With the Soviet-style MSM we have in this country now (John Edwards case in point!), that can be a real problem.
There could be a video of Obama sitting at his desk rubbing on his birth certificate with a pink rubber eraser, and no paper in this country would print it except the National Enquirer.