That's where the AGPL comes in. The GPL(v2) does not require eg Google or Facebook to release any of the changes they've made to the Linux kernel. That they do so is not because of a legal obligation to do so. The "to get parts" thing is the relevant detail to be very specific on. If those parts are a binary that is used, then the GPL does kick in, but for distributing source code that's possibly derived, possibly not covered by copyright, it's not been decided in a court of law yet.