Many bare metal or RTOS systems consist of a handful of statically linked programs (one or two bootloaders and the main application), many companies would rather find a non-GPL library rather than open up the rest of the system's code. Sometimes a system contains proprietary code that may not be open sourced as well.
1) you may not have the right to open the rest of the code on the system
2) although you make money when you sell devices, it also makes cloning trivial