Agreed. It’s absolutely unclear that the ThemeForest Wordpress themes are not compatible with WordPress.com. When a service is advertised as…
“FullScene is a professional, multi-layout, ultra flexible and customizable portfolio WP theme …”
... it would stand to reason that it should work with Wordpress. That’s what it states. There is no distinction in the description on the detail page for the theme between hosted with Wordpress.com vs. hosted through another service with Wordpress.org.
Sellers on Envato should know that a general confusion exists between users of Wordpress.com and Wordpress.org (this thread is evidence of it) and should simply point out that their themes are not compatible with Wordpress.com. It’s false advertising to sell under the umbrella and brand of “Wordpress” while not being explicit about the distinction between Wordpress.com and Wordpress.org. This should not be a deceptive game of insider baseball, where folks take money for their product and then expect the casual user (and, actually, the majority of Wordpress users) to know of this distinction.