Bill of Rights vs. Federal Law

Shouldn't our freedom of speech take precedent over a federal law that "bars harassing, intimidating or threatening a foreign official in the performance of their official duties."? It seems quite clear to me that the framers our constitution felt that it should. So why is the US Attorney's office prosecuting someone for exercising their rights under the First Amendment? Are we so concerned with not embarrassing international leaders that we are willing to give up our rights? Sounds unAmerican to me.

Here's a link to the story:

China heckler at White House charged