A disclaimer stating that scams are not banned doesn't absolve the forum from responsibility of being a vehicle for fraud.
By that same logic, if your Bitcoin node relayed a fraudulent transaction it received it would be a vehicle for fraud, therefore it's safe to assume every Bitcoin user running a node is just as guilty as theymos as undoubtedly every Bitcoin node has relayed a fraudulent transaction.
We're not discussing logic or how things should be, had we our way. We're talking about US laws. US laws do not allow US person to run websites where people sell CI drugs, or promoting unregistered securities. This is basic stuff.
Thankfully the law doesn't work that way and there are laws in place designed to protect website owners from the actions of their users, which is why craigslist's owners aren't in jail for the actions of people like the Craigslist Killer.
If Craigslist operators knew the ad was placed by a murderer, were informed, on multiple occasions, that the guy was killing people, and refused to take down the ad? You can be absolutely certain they'd be jailed.
Further, if most of the ads on Craigslist were made by killers, putting a disclaimer along the lines of...

or, rather, "We do not remove ads by likely killers. Thus far, it appears that roughly 95% of these ads have proven to have been placed by confirmed murderers. Use your head." just wouldn't cut the mustard

The forum has instituted a policy to generally not look into the various deals that people are considering to make. As a result it does not know if something is a scam or not. A similar thing can be said about unregistered securities as the forum does not verify that something that could be considered to be a security is properly registered.