Stallman is perfectly happy avoid using your service and resources. His issue is that he doesn't have an easy way to tell whether or not he *should* avoid you.
Sure he does. If Stallman wants to know whether a site's Javascript is under the GPL, he can just look for the text of the GPL included as a comment on the webpage or in the included JS file; since you're supposed to distribute a copy of the GPL along with any GPL'd code, right? I mean the requirement that the license is distributed along with the program is right there in section 1 of GPLv2.
It'd be fairly trivial for someone to put together a browser extension that refuses to execute any code that doesn't come with a copy of the GPL attached.
If the fact that nearly every HTTP response would end up at least 17k larger if the idea were to take off is too large to swallow; then just include a link to the GPL in one of a page's tags. It'd be just as easy for a user agent to check that as well.