From Julian's essay:
"Let's suppose even further that LibreJS succeeds so much that it causes a
large portion of the Web to release scripts under libre licenses and document
the licenses in a format LibreJS can understand.
It seems great on the surface, but what follows from this is that software is
still being silently installed into our browsers every day. The only
difference is that LibreJS thinks the programs are libre."
Right, and this is the only problem that LibreJS aims to solve:
http://www.gnu.org/software/librejs/manual/html_node/Disclaimer.html#Disclaimer
Maybe a different tool could figure out if the free script has changed since
the last time you ran it?