What? Cops can't tag you while they're moving too. First, i don't think the gun is connected to their speedometer, and second,, isn't it kinda dangerous driving an automobile and aiming your radar gun around (and looking at the speeds on the radar gun)?
I don't know for sure how it works in the US, but i know up here in Canada (well, i should say 5 yrs ago, not too sure what it is now), it wasn't possible to tag you if they were moving.