Was checking out a theme today, which led me to the GNU GPL page. I decided to refresh my memory and found this useful nugget:

Does the GPL allow me to require that anyone who receives the software must pay me a fee and/or notify me?

No. In fact, a requirement like that would make the program non-free. If people have to pay when they get a copy of a program, or if they have to notify anyone in particular, then the program is not free. See the definition of free software.

The GPL is a free software license, and therefore it permits people to use and even redistribute the software without being required to pay anyone a fee for doing so.

So, this means I can legally say “no” to any attempts at free promotion by unscrupulous theme authors by removing unrelated business links from their themes, if they are GPL’d.