It could be, if it ran in something that you got when buying the stock mouse.
It does not?
It does not?
I get your point but I'm confident that if I had to lawyer up I would win this one :P
Your argument is more one of philosophy / law / BS than something anyone would take as true.
To be less academic: I don't think there is a mouse in this world where you can actually modify the firmware, because using flash memory for something this dirt cheap and with a functionality that hasn't changed for decades is out of the question, so this is probably the only way to do it...
(how I know: I managed to softbrick one - and reflash to working condition.)
Does the person buying a Civic capable of running 10X care? No. It's a 10X Civic. They've already crashed.
Many cheap mouse sensors these days are ultra integrated to the point they often even lack USB pullup resistors. If they could be reprogrammed to, say, add a pixel dump command that very few of them implement, that would be very interesting.