*Sigh* Let's explain this using one of your own examples:
insanedrunkenbaby wrote...
In tennesee it is illegal to sleep and drive at the same time.
Imagine that a cop pulls someone over because he sees that they're driving erratically, and it turns out that the driver kept falling asleep.
The cop takes him to court for this.
The sleeping driver turns up with a brilliant defense lawyer, and gets off scot free because technically it isn't illegal to sleep at the wheel. Because of this loophole, a dangerous driver who can't stay awake in his car gets back on the road with no repercussions.
To avoid this circumstance in the future, the state passes a law making it explicitly illegal to sleep while driving, thus an apparently "Stupid" law arises.
Take that example, and mold it to fit with most of the strange laws that exist and you will probably see a similar story behind it. It is, as Flaser said, all about precedent.