Revolutions are always illegal. The American Revolution was very clearly illegal as far as England was concerned, and the US Civil war was illegal to the Union. Even the Founding Fathers, some of whom certainly believed the right to revolutionary violence was necessary and healthy for a free state, would not have accepted the right of the people to string them up.

But your second sentence isn't accurate. The US is almost unique in the degree to which discussing the violent overthrow of the American government is not only tolerated, but held sacred by many people. From right wing talk radio to the Tea Party to Republican political ads during the Obama administration to NRA rallies, intimating the need for revolution and "Second Amendment solutions" is common enough that it may be the oldest meme in American culture.

Of course, there is a line beyond which it becomes conspiracy to commit murder, and then it becomes illegal, which is to be expected as every state assumes its own legitimacy. But the chilling effect you seem to be trying to suggest doesn't really exist as far as I can tell.