I asked myself the same. Makes no sense at all from a conceptual point of view so the only explanation I have that it has to do with the way the whole scrolling logic is implemented in Cubase and that it would be a pain code-wise to completely rebuild it. So they came up with some sort of a compromise that isn’t really helpful.
I mean, the main concern usually is the fader area. Of course I want to have the others parameters locked as well but I could live with a solution that locks everything in the faders area. But the opposite is the case.