HR437, "the Federal Restricted Buildings and Grounds Improvement Act of 2011" makes it illegal to protest in the vicinity of anyone who rates a Secret Service detail (even if you aren't aware of the person's presence), thus sparing politicians and VIPs the ugly and unseemly spectacle of having to confront voters who disagree with their policies. Only three Congressmen voted against it.
On top of that, the punishment can be pretty severe. You can get up to a year in jail for being found guilty of these things, and that jumps up to 10 years if you are carrying a "deadly or dangerous weapon."
As Amash notes, there are legitimate safety concerns to be aware of, and there are issues with doing something that significantly impedes government regulations. But it's really not difficult to see how this bill could very, very easily be stretched to be used against those doing standard protesting against significant political figures.
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.