I have no problem with the regulation and inhibition of any forum by an individual or group provided that the said forum is essentially the sole property and creation of said private individual or group.
If you publish a newspaper, you get to print what you want (within the bounds of laws against libel, the improper use of copyrighted intellectual property without consent, government secrets, or inciting speech, etc...). You also get to exclude what you don't want to print. That's not censorship, thats private discretion, which is a cornerstone of free speech. In other words, free speech doesn't just mean that you can say or publish whatever you want, it means that you don't have to repeat or publish something just because some other person or agency wants you to do so.
The confusion occurs when people believe that THEY THEMSELVES are publishing on Mudcat. The truth is that while they are doing the writing, it is Mudcat in the persons of Dick, Max, Susan et al, which is doing the publishing.
So in the current context, those who submit these writings are enjoying the full excersise of their rights by being able to write what they want and to freely send it to another party with a request that it be published by that other party. On the other hand those associated with Mudcat are enjoying full excercise of THEIR free speech rights by having descretion over whether they actually do publish it. The fact that Mudcat is currently choosing a priori to publish just about anything that comes in does not imply a waiver of that basic privelege.