When linking, symbols pointing to addresses are involved. The main program calls functions in the library, and you end up with a whole program. On the other hand, a bootloader just does its thing and then jumps to a fixed address in the main program. The main program doesn't call anything in the bootloader. (Note, the Linux kernel has a special clause that explicitly allows the userland to programmatically interface with the kernel.)
I realize this sort of depends on the microcontroller toolchain. I'm familiar with a few, but not much with STM32 stuff. If the toolchain is dumb and implements the bootloader in a way that resembles linking, i.e. making it possible to resolve all the symbols in the bootloader from the main program then it could very well make the bootloader subject to the GPL.
I did a presentation[0] on this about a month back. If we want these situations to stop we need to work to inform the public and work towards getting people that care in positions of power.
[0]: https://www.scottrlarson.com/presentations/overcoming-survei...
That community is where open source tech tends to come from.
I think a valid view (not saying it's right) of this is: "some dorks in a country I don't inhabit gave away valuable code. I'm going to use it and I don't particularly care about what they think about it"
How much do you care about Chinese law? does that make you sketchy?
It's an easy thing to avoid the costs associated with complying with FOSS licenses because they FOSS people don't come after you. So instead of having a person on staff who is making sure the company is compliant and updating the web site with source as it changes etc etc. You just don't do any of that and not only do you save money not paying that person you don't have to bother the web site people every time you do a software update. So from a manufacturer's perspective it's a win.
That said, Chinese vendors selling to Chinese customers following Chinese law. All good. Chinese vendors selling to US customers and not following US law, that's bad. I don't see how Chinese law would be part of that discussion.
What makes them particularly sketchy is the hypocracy: they fully understand the concept of IP and they are using/abusing it to try defend their semi-walled garden.