Yeah, Subaru can have the Starlink disabled pretty easily by removing, essentially, a module behind the head unit. The only problem is that module also sends power to the front speakers. There's been workarounds created, but it's just asshole design at its finest.
Yeah, good point, perhaps there is some engineering rationale for having them powered the same, so that the speakers are guaranteed to work as long as the Starlink does.
Car manufacturers are required by law to offer the same tools that dealers use for independent repair shops to repair their vehicles.
Some cars are more programmable than others. BMWs for example you can change pretty much anything about the car. But most cars aren't as modifiable as them.
I assume that it isn't much technical knowhow to take a pair of wire snips and snip the power or antenna to the OnStar services. Least I assume that is what would be done
Its not illegal, but if someone starts selling these (car modification) services, and your car crashes, the authorities might hold them liable (even if its totally unrelated to the crash).