Seeings you are insisting on continuing this discussion I have split it out so as not to take the other thread off topic any more than it already is.
Themeforest predominately sell WORDPRESS and CMS themes, I specifically said "forum theme world"
Designers are forced to submit to Theme Forest's licensing agreement when uploading to that site, most weigh up potential sales against their copyright and designer rights and come down in the favour of potential sales. That does not however mean that such a model is right, or the only way to do business.
Most forum theme designers are not under any such pressure to conform to a 3rd party site's will as they sell directly, and thus do not allow you to do whatever you want with a theme design.
If I was required to give up the rights to my designs by uploading them to a 3rd party site, I would personally choose not to, as would the majority of professional designers I know. Which is why you do not see any "theme forest" business models in the forum theme world, the potential sales are just not there to warrant handing over all your rights to a design.