Software. Hardware. The difference is not relevant.
I also can't easily modify my toaster to fry sausages, because of the toaster company not releasing the design schematics and moulds. It's not free hardware!
The toaster lets you open it, the toaster lets you modify it, it has standard screws and doesn't explicitly lock you out. It gives you no help in modifying it, but you are permitted to do so, and there are no mechanisms in place to stop you from doing it. If someone were to make a tool that lets you modify the toaster, that would not be illegal, and I highly doubt the toaster company would take issue with it.
I also can't easily modify my toaster to fry sausages, because of the toaster company not releasing the design schematics and moulds. It's not free hardware!