Well you may be right IF and that is a big IF those towns on Hwy 301 in Florida do that. But they don't. First, there are big, and I mean big signs about 1/4 mile that clearly states reduced speed ahead. Then there are signs, not that big, that tell you the new speed limit ahead. Then when you enter the new speed zone, there are gigantic signs telling you the new speed limit. These towns are not speed traps. They aggressively enforce the lower speed limit through their town. I don't blame them. Otherwise, people would be speeding 50+ MPH with pedestrians on the sidewalks and kids walking to and from school.
I drive Hwy 301 in Florida frequently, and have never had a problem. The people who get a ticket very well deserve it.