From the vagueness of your statement, it sounds like these forums fall into the second category of being up to no good in the first place.

aka: "if you aren't doing anything wrong, what've you got to hide"

Not necessarily "wrong". For example, you could have an internal company wiki which would include links to competitor's site.

Unfortunately, public protocols like HTTP need to manage communication between parties that do not trust each other. This is not a legal matter, so there is no necessity to establish motives or plausibility. A secure protocol is charged with limiting the damage any malicious party using the protocol might do. This addition to the spec is contrary to that fact.

Allowing a site to scrub its outgoing connections assumes a certain level of trust, which on the web ought not exist by default.