The Nixpkgs community is internally diverse, but broadly values both "purity" and pragmatism. You can see debates and compromises play out in PRs all the time, or read traces of such careful weighing in the source code of Nixpkgs itself.
For the record, the Nix community's largest public cache doesn't cache binaries of proprietary software because doing so would be illegal— the public doesn't generally have the rights to redistribute proprietary software.
The phenomenon of having to compile free software from source via Nix typically happens when free software depends on proprietary software (which is common on macOS). Maybe this could be ameliorated on a technical level, but I think it's mostly historical accident and ease of implementation that got us to the current situation, where the whole dependency tree has to have a free license for something to make it into the binary cache.
For the record, the Nix community's largest public cache doesn't cache binaries of proprietary software because doing so would be illegal— the public doesn't generally have the rights to redistribute proprietary software.
The phenomenon of having to compile free software from source via Nix typically happens when free software depends on proprietary software (which is common on macOS). Maybe this could be ameliorated on a technical level, but I think it's mostly historical accident and ease of implementation that got us to the current situation, where the whole dependency tree has to have a free license for something to make it into the binary cache.