> I don't understand why any software other than their own should be entitled to use those resources
That's not a genuine argument, nobody "feels entitled" to anything. Bambu made a deliberate choice to architect the product this way, deliberately placed themselves in this gatekeeping position, and they're deliberately working towards removing any other form of access to our hardware.
> they're deliberately working towards removing any other form of access to our hardware
Maybe I'm mistaken, but I don't think that's what is happening. They aren't doing anything to block OrcaSlicer or any fork from working with the printer using LAN-only mode. It's only if you want to use Bambu Lab's servers for essentially a remote-access solution (which, by the way, kind of defeats the privacy-oriented purpose of running some of these forks) that they're saying you should use their own software.
Thought experiment: the core of macOS (Darwin) is open source. Does that mean everyone running Darwin or a fork of it should be able to use iCloud services for free?
All this outrage essentially sounds like "since Bambu Lab's slicer is open-source, the open-source community should be able to point any slicer at Bambu Lab's servers to get free remote monitoring services". And I don't think that's right.