Hate Crime is really more of a legal definition, and exists in other countries as well.
Not saying it's right or wrong (for the sake of discussion), but the rationale behind it is that you are making an additional psychological affront against not only the victim, but also the group the perp is attacking.
Another way to think about it is that you can get in trouble discriminating another group in a bad way,
even without the use of violence (i.e. saying you hate all gays at work explicitly). Therefore a 'hate crime' takes into account the crime that occurs regardless of the discrimination, PLUS it also takes into account the said discrimination.
The way I personally see how it could be severe in a legal sense, is that it pretty much establishes the crime as having a 'malicious intent' (vs. a spur of the moment, or accidental). Having malicious intent usually is recognized by our and many other legal systems to be much more severe.
http://en.wikipedia.org/wiki/Hate_crime