You're talking about a sign with practically no legal meaning.

This discussion is about a law which does have legal meaning.

To fix your analogy, it would be "If there's a law that says only explicitly authorized cars are allowed in a car wash; else 5 years in prison, and I have a car wash and say 'only red cars' ...". Of course, that still doesn't properly capture this since the intent of the law also matters and that law would be senseless, and so interpretation would be less obvious.

The better analogy is the one below about breaking into homes being illegal, but what if you happen to have a key that fits the lock? (though that also is a bad analogy in its own way).

Basically, making physical analogies for technical matters is rarely correct. It's often the best way to convince non-technical people of a matter without them needing to actually understand it.