Most themes that aren't featured on the repo, are usually 'premium' themes, and more often then not, they have their own support forums or support system. If they don't, they really should. The restriction of having only themes featured in the repo be allowed in the official support forums is a great idea.

I believe that any thread requesting for code to be decoded should be referred to an official statement in the codex (or a post on the forum) which states the danger of such code, and why users shouldn't use themes that have it, etc...

Ultimately, this would allow for a clean-up of the forums, and for users to concentrate on helping each other with supported WordPress things, not junk themes they found on a warez site or other...