Well, it is pretty rare for the US, though, unfortunately, if things continue down the road they are currently travelling, I can see it becoming more common. That said, however, you only have to go back a few years to realise that in the US, assassination of political figures is a fixed part of their history. If you take people like Abe Lincoln or JFK, both of those were killed (if memory serves) because their policies didn't sit comfortably with certain groups.
What makes this so horrific is that I don't think it's an assassination, I think it's the result of a nutcase with a gun, and trying to brand that nutcase as Libertarian, Republican, Tea-Party or anything else is kind of missing the point in my opinion.
