So if they suddenly change the speed limit to 10mph on the M1, should I abide by it solely because it's the law, even though it makes no sense?
The real issue with cameras is that they changed the way speeding laws were enforced. When actual police officers enforced the law, they used speeding as a means to punish bad drivers, they used discretion, that's why points used to mean something. A policeman wouldn't (and still won't generally) pull you for going too fast, they'll pull you for driving like a chump. Although the law was not written in a discretionary way, the old style enforcement was.
When cameras were introduced, they stopped the discretion, they suddenly made it black and white where it never was, and it's become apparent that the law is an ass. The government's determination to carry on down this path because there's money to be made has compounded the problem.
Safe speed has nothing to do with the number on the dial or at the side of the road, it's far, far more complicated than that. The police used to recognise this, and most of them still do. It's the scamera partnerships that don't.