In so far as they would consider abolishing freedom of association: they are going against their oath to uphold The Constitution of the United States.
It suggests they don't consider their promise to the public when discussing or possibly deciding legislation. It suggests they think of themselves as rulers instead of public servants.
For example, if a corporation were to discuss defrauding the public as a way to solve a particular problem; people would generally be upset on finding out.
With a company you might have a choice whether to do business with them. But with government, since they have a monopoly on force (fines, prison, etc), everyone is subject and there is no other (safe) choice. People who don't have a choice about something they disagree with: well, they sometimes resort to more physical methods of argument
I don't advocating violence over something that's just being discussed: but it's perfectly reasonable why threats of violence would be a reaction in this instance.