For some, open source software isn't just seen as a "nice to have", but the only way to avoid a society where freedoms are limited by dependence on software which works against us by design.
Even if nearly everyone uses GPL for their JavaScript, you still need to go through this process for every site that may use non-free software, which is a royal pain.
Perhaps a "better" solution is to fight to have browsers check the license of JavaScript before loading it (either prompt the user, or have a browser setting for acceptable licenses), but that's a much harder battle than promoting JavaScript-less web pages, especially since the user has control of blocking JavaScript today.
https://www.gnu.org/software/librejs/
https://www.gnu.org/software/librejs/free-your-javascript.ht...
It can be a bit fickle. I recently tried to make code comply with it and it didn't initially work because I was using a non-standard version of the ISC license with one different word.