But then again isn't all client-side JavaScript open source?

No, you can see the source, but thats not the same as "open source".

this is an important distinction. Everything on the web is copyrighted and you can't just go and grab images/html/css/javascript without permission from the copyright holder.

And view source isn't going to show you much when looking at sites like Gmail where the code is generated by a compiler that makes it as obtuse and unreadable as machinely possible.

Exactly. Think of it like a book. You can see the source code of a book (i.e. the individual letters), but you cannot copy it (without permission)

GPL requires that users have access to the preferred form for modifying the work. If you're using coffeescript or a state machine graph or whatever, the generated javascript is more like object code than maintainable source.