It's not clear to me why I shouldn't tolerate closed-source Javascript that I can nonetheless inspect, archive, and edit on my own machine, but I should tolerate a closed-source SaaS backend that I can't inspect, archive, or edit.
Of course proprietary software limits freedoms, but does it limit freedom more than serverside logic? There are equal legal restrictions on both codebases, but I can't even exercise fair-use freedoms with code that I literally can't touch.