So many people complained about not being able to run their on firmware on the TiVo that it caused the GPL to be updated to version 3.

While Turing machines are universal, there are practical limitations of the hardware. A tiny embedded microcontroller with kilobytes (or less) of memory is not an attractive target for customization or repurposing. Today it is probably easier/cheaper to simply buy a Rasberry PI or similar.

Also, some companies understand that they are in the business of selling hardware and don't particularly care what you do with it.