The problem isn't exit, it's entry. The requirement is that it be a valid, meaning unexpired, passport. So, technically speaking, any US Citizen that isn't carrying a current passport when they attempt to enter the US is committing a crime. But note all the "unless otherwise authorized" language. There are many explicit exceptions to the rule (e.g., the "Enhanced Driver's License" that works when coming in from Canada) and also, as you point out, the section doesn't give a penalty.

I agree with the other posters here: unless there's something else going on (e.g., some other illegality or conspiracy), the chances of anybody being bothered by this is vanishingly small.