Let us suppose that Microsoft wishes to embrace and extend the Linux kernal. I don't care what exactly they want to do with it, but they do not wish to share their code with anyone else. The Linux kernal is of course protected by the GPL, which ties a contract agreement to the fact that the GPL can use copyright law to deny you the right to use GPL-protected code. (That's key; if you want to modify the code, you must fulfill these obligations. If you do not modify and use the code, you do not need to fulfill the obligations.)