Also, in most cases I'd assume you didn't get useful Busybox code out of the process because nobody ever needed to modify it. (As opposed to the Linux kernel, where vendors frequently add drivers or other patches to customize it to their hardware.) Even in such cases, the GPL still obligates vendors to distribute their code, which at least includes the Busybox .config the vendor used, to help people who want to create modified firmware images.

That said, I wonder sometimes if some projects should take the approach used by the UPX license: GPL, but if you make absolutely no modifications and just compile and ship the thing, you don't have to provide source. On the other hand, that would make it extremely difficult to tell the difference between someone using an unmodified version and taking advantage of that provision, and someone shipping a modified version in violation of the license. Better to just require source in all cases; it honestly shouldn't burden a vendor that much to stick a tarball somewhere and point to it.