You are correct. This is a violation because this is members of Congress threatening an organization with law if they don't change their speech/message.
This is not you or me threatening NOT to listen to the Ditzie Chicks.
"Congress shall make no law" is the point of the 1st amendment, and these congresscritters failed miserably.
Is it possible to have the SCOTUS rule on a "threat" intended to stifle free speech or does it have to be a finished law?
Hannity on this story now