Also not a lawyer, but my understanding has always been that a license grants permissions, not limits them. No license means no permissions granted. Most sites have terms that you agree to (by posting to the site) that tell you what they may do with your content, and I don't think a license you tack onto it can change that (though it can grant permission to others).

As for scrapers and such, they were never granted any permissions to use anything. They just don't care. A license is also unlikely to change that.

I think licenses on posts are pointless and tacky, personally, but I could be missing something.