It is not settled that add-ons have to be GPL. The add-ons are python programs that do not contain or link against Blender code. Instead, they are called by Blender itself (and can call Blender python APIs). It's not much different from a Linux userspace program, for example.

There are reasonable arguments either way. For example, is an add-on for an electronics device that directly interfaces with the motherboard considered a derivative work? How about an accessory for some kind of tool? Like a plug in, they are useless without the original device, but does that make them derivative works that need permission to be distributed? We'd argue not for copyright of the designs of physical goods, so why would it be different for software?

There are clearly arguments either way and this isn't a settled matter.