Free speech is an extremely popular cultural ideal in America. However, there is a distinction the courts have made over the years that prohibits a call to action for a violent act:

https://en.wikipedia.org/wiki/Imminent_lawless_action

ISIS recruitment obviously violates this. If the right wing propaganda violates this, it is also illegal.

I also don't think America has (or believes it has) a right-wing terrorism problem. Sure, any bit of terrorism is a problem, but there seems to be about 20 murders a year attributed to right-wing terrorism in a country of 300+ million people. (If you live in a country of 15 million, that would be like 1 person/year). For comparison, cities like Chicago will have 20 people murdered in one week in the summer.

My point is just that you can't ask the American people to part ways with one of their most cherished political ideals, to fight a problem which is not viewed as much of a threat.