I’m talking about sites like GPLDL that redistribute “premium” paid themes for free. That’s not cool at all, that’s just stealing.
That's not correct. GPLDL only shares themes and plugins that were published with the the GPL licence. If a theme or plugin was released with something other than the GPL licence, than you wont find it on GPLDL.

Sharing or distributing themes or plugins released under the GPL licence is not stealing. If you release a theme or plugin under the GPL licence, whether it's a premium or non-premium theme or plugin, you have accepted the fact that anyone who acquires the theme or plugin is free to distribute it as they see fit. This is spelled out clearly in the GPL licence:

GNU General Public License