According to the official docs all Kernel Events should extend KernelEvent but PostResponseEvent doesn't.
I also have a valid use case for it, which follows:
I'm storing the last accessed page by the user in the session, so I've registered a listener on kernel.terminate for that, but the request's info should only be stored, if it's a master request, however without thePostResponseEvent extending KernelEvent I have no way of knowing that.