There are advantages to both.
If you try to restrict readership, you can hope for more openess amongst those who are admitted but the information is not getting out to everyone who needs it. Plus the boards that try this often end in tears when people fall out.
It's also possible to get it wrong. One warnings site for male escorts needs a password to read the posts, but not to read the comments... which is where the actual information is.
There is a middle ground (where everyone can choose who sees a particular post of theirs: anyone, or everyone who's a member of the site, or everyone they trust, or a subset of the people they trust) but the message board software that implements these sorts of features doesn't seem to have been adopted in the sex industry yet.
Personally, I quite like the idea that the nasties can see that they're being discussed and warned against.