Non-excludability isn't just about the law; it's about practicality. It might in some system be legal to prevent fire services from putting out fires in houses that haven't paid for it, but that would be an impractical system.

Similarly, critical software intended for developers is impractical to deny to them; doing so has some serious negative effects on the production process (hard to get good feedback/PRs from customers, for example, unless you expose the source to them to a level that closed-source manufacturers find dangerous to their business model).

WRT the original commons, you can make such things work in a tight-knit community that can enforce social norms (which, by the way, would take on a lot of what are now considered "government functions" in a modern society), but in a larger capitalist economy with actors that aren't inside the community, the government is the only actor that has the authority to enforce public ownership of the commons.