One of the great strengths of America's constitutional and academic systems is the latitude they give towards free speech. We allow people to advocate overthrowing the government, so long as the call is "abstract" and no concrete steps are taken to effectuate the call. We allow professors with such manifestly anti-American views to continue teaching, unhindered and unmolested, because that's what academic freedom means.
So when this George Mason professor seemingly promotes overthrowing the American government (and, perhaps, watering a tree with President Obama's blood), know one thing: he absolutely has the right to do this. He will and should face no official professional sanction for declaring himself an enemy of the lawfully constituted government of the United States. Whatever we think of a university system which is inhabited by such radicals must take second chair to our commitment to free speech and academic liberty.
UPDATE: The professor's post has been updated, removing the part about watering the tree of liberty with Obama's blood.