Proprietary software depend on copyright to prevent owners, which today often mean property owners of physical objects with DRM in them. Property owner is deprived of all rights that personal property is normally given to their owners. That power is instead taken by those that produced the object.
If sale of property actually follow traditional first sale principles and gave the new owner rights which used to be naturally aspect of private property, then practically all freedoms under Richard Stallman definition is given. You don't have to give away the apple (in the classical example), but once sold you can't have copyright or patents preventing the new owner from using the apple to grew a orchard.
Noone guarantees you that a year from now you will be able to find anyone who is willing to fix your plumbing. That does not mean you might as well have proprietary Microplumb products installed in your house that noone can repair or extend without permission from Microplumb Inc.
You can observe that most software is leased, not sold, thus restrictions apply. And indeed - the same holds in physical world; if you're the house/appliance/car, you might not be able to use it the way you like, or get whoever you like to repair it. But that's precisely the problem! Renting instead of owning has serious drawbacks, and Stallman is fighting against those, trying keep us at something more akin to personal ownership model.
If you have any open-source project, you've probably received a lot of e-mails like that.