The issue is not that they sell 3d printers with proprietary software. The issue is altering the deal _after_ you have purchased the printer. People bought a printer that was open. Then suddenly the company changed their mind and pushed an update that made it non open. And when people try to restore the software to the state it was when bought, the company fights such attempts with dmca requests. If I bought something it's mine and I can do whatever I want with it and run whatever software I want on it.

Yeah this practice needs to end. You buy a stack of hardware+software that does X.

Imagine if pizza consisted of software and hardware and you only bought hardware but software could be changed by dev/seller. Now your pizza shrunk in size, changed taste, or could only be eaten by a fork that is supplied for free by the pizzashop, otherwise special chemical compounds would make it disintegrate if you'd try to eat it using your hands or anything else. Technically you still have that pizza you bought...