The short answer: this has been suggested before, and turned down unanimously by the moderators. Your analogy of protestors at G8 and similar is faulty: we are not government. Government (at least, the US government) can only infringe on free speech in certain fairly well defined circumstances. A private individual or private corporation is under no such restrictions. We are not required to allow “free speech” in violation of our rules.
The same is true of a movie theatre, say: if someone jumps up in the middle of CHARLIE AND THE CHOCOLATE FACTORY to scream obscenities, and persists after management has asked them to please shush, they will be firmly escorted off the premesis. They will not be given a little roped off area in the theater.
If some walks into K-Mart and makes lewd suggestions to the clerical help, they will be invited to leave, they will not be told to restrict their activities to the toy department.
Imagine you have a party in your house, and one of the guests yells racist comments, breaks the furniture, and takes a shit on your living room floor. So, for your next party, you restrict them to the tv-room?
Say a company has fired several employees: one for fighting, one for stealing from the company, one for making obscene suggestions to customers, all after several warnings. You’re suggesting that the company should provide a special area for these folks to do whatever they want, so the others can watch in amusement?
Look, in the first place, there aren’t that many bannings. In the second place, we now have the tool of suspension in place, to give people a time out and see if they can come 'round.
There are two types of bannings that occur:
= One-time drive-by spammers, porn-peddlers, and the like. I presume you’re not suggesting that we reply to invite them back to a special forum for spamming?
= Members and guests who can’t play by the rules. Sometimes (rarely) the offense is egregious (like threatening bodily harm to another member, for example.) More commonly, the offense is a consistent, repeated pattern for which they have received ample warning (such as personal insults in inappropriate places.)
Why on earth would we want to provide such people with a forum to rant against us? There are plenty of other forums out there, some of them having no purpose other than hatred of the Straight Dope. Let them go where they will, but not here.