The 'P' taint flag exists because upstream kernel maintainers aren't interested in dealing with bug reports where the bug may have been caused by proprietary code loaded by end users, which often can't be examined or copied even for the purpose of discussing the bug. Total waste of time, so the flag makes those cases obvious to anyone reading the bug report. The user can be told upfront to remove the proprietary code and try to reproduce the bug again.
End users can combine proprietary code with GPL code if they want to, because end users aren't bound by the GPL unless and until they distribute something covered by it. You don't even have to accept the terms of the GPL just to run the program[1].
A company distributing a proprietary module for the kernel may or may not be violating the GPL, depending on what it does and who you ask.
In contrast to the module stuff, GRSecurity is a set of patches to very low level parts of the Linux kernel itself. It could never be licensed in a way that prevented the patch set or the resulting patched kernel source/binaries from being covered and distributed under the GPLv2. If that were the case, the kernel would effectively not be covered by the GPL at all.
