It's typically done in "problems areas" where a small number of people drive 25-50% or more over the limit, but it's extremely frustrating, because it forces drivers to drive way below the actual limit when crossing the speed bumps - especially if they're the bolt-down kind. i.e. the effect is to cause irritation for ~99% of drivers in order to punish a tiny minority.

[1] I'm sure it happens elsewhere in the US, too.