Because in almost all other situations of life you are required to obey the law even if you aren't aware of it. For a different example - cyclists aren't legally required to pass a driving test, yet are required to be familiar with, and obey all rules of the road. You couldn't argue in court that as a cyclist you don't have to stop on red because the law doesn't require you to pass a theory test before cycling. I find the situation similar - you should make sure not to go over the speed limit, even if the law doesn't require you to carry a speed measuring device with you.

fair points