It's true that a community exists by the virtue of its members. Without members, there is no community.
Yet that does not mean that everyone can just do as they please. And there is also no free speech here. Free speech implies you can say what you want, when you want, how you want.

We don't allow people to be rude, we don't allow them to advertize their wares outside of designated areas, we don't allow discussions of a sexual nature, we don't allow threats, ... There is a whole of of topics that is not allowed. In other words, there are restrictions. This has always been the case, and is no different from other online or real life places where people gather.

Besides, this is not even a free speech issue. Just because you have freedom of political speech in the US does not mean you are allowed to spraypaint your opinion on the national monument, congress or the white house. There are places on this site where you can express your opinion. We decided to not allow it in between posts about shaving.

And yes, 99% of the people will follow the rules. There will always be a couple who don't want to listen. We'll have to deal with them. And while they can indeed come back under a different alias using a proxy, that is really no different from other people we have to deal with, such as spammers. We would have the same issue with anyone who does not want to abide by the rules on advertizing.

But even when we allowed sig lines, some people used them inappropriately, and we had to deal with them.
this is really no different from what we would have to do if we implemented your proposed sig line rules.

With any change that is made, usually the vast majority likes the change. There will always be some who don't like it. If that should prevent us from doing anything, we would get nothing done and we would still be on the yahoo group. When we switched to the classifieds system, most people liked it, and a vocal minority hated it. All the arguments that were made so far applied to that change as well. People could still break the rules and conduct business in the main forum, and if they got banned they could get back using a proxy and a different nick, etc etc. The same arguments can be made with every change.