Got it the first time. If Gary Trudeau tried to submit anything to a paper I owned I would not accept it either. I gather you like to waste your time sending them the email knowing that they are not going to publish your work. If that's the case you are either a masochist or you are just harrasing the publisher.
I agree with you on that point forcing somebody to publish something in their newspaper would be violating their free speech.
If he was using legal (meaning lawyers) or coercive means to prevent the strip from being written, distributed, or published elsewhere he would have violeted the autor's free speech.