My bro has one of those and it does work, if a little inaccurate. +- about 4 mph, not too bad. It's a little hard to get a track on an RC though, even the Baja needs a couple passes to get a reading... but it may just need new batts.
I've wondered about them myself but though they were prolly really off, i know the decible meters from walmart can be up to 17 decibals off! wich makes them worthless imo, but this might be something worth looking more into.
Yea cops DON'T have one, cuz there's no point TO have one lol unless you are the drummer from def lepoard and you need to check your sons pitching speed while videotaping it of course