What about insurance companies?

Here in the US, when I insure my car, I've always found a clause in the contract which states that I'm not covered for theft, if the keys are left in the ignition.

Now, it's stupid to compare car theft to using an open AP, but clearly, people shouldn't entirely rely on laws to protect them. Laws, much like locks, serve mainly to keep honest people out. As one commenter pointed out, WEP is a joke from a true security standpoint, but so too is my ignition lock, and no doubt the locksets on most of our homes. The point is that each is a layer of security, which, in most cases, is good enough to deter all but the boldest criminals.

In the case of a wireless access point, the physical realities are that when one broadcasts a radio signal into the air, there is always the possibility someone will have a suitable receiver, capable of picking up and using your signal. If you don't want someone to do so, the burden is most certainly on the transmitting party to calibrate their instruments correctly.

If broadcasting unencrypted signals with only the threat of the law to prevent people from receiving them really worked, would satellite TV carriers do so?

If legislation is appropriate in the case of Wifi at all, it should be placed on manufacturers, requiring them to forcibly enable WEP, at the very least, on all consumer-grade APs. To apply the law so broadly, that anyone connecting, even accidentally, to an open access point, would make criminals out of so many people as to make the law laughable.