Because the point is to slow the traffic down, not to extract revenue from the peasantry.

Same as the difference between an obvious speed trap and a "gotcha" speed trap.

But does it slow people down? I doubt it has significant effect. Very few people are going to be going over 80 a few meters in front of a 50 sign. You essentially are only catching people who are doing close to double the upcoming speed limit. Those who are willing to do that should be getting much more severe punishment than a speeding ticket.