That simply isn't true! Even in California, where anti-moonlighting clauses are very difficult to enforce, there are circumstances (conflict-of-interest being an important one) where you can't work two jobs. In many other states,
all anti-moonlighting clauses are considered "reasonable" restrictions on your right to work.
Sorry for the stridency, but the logic that carries you to that conclusion is dangerously faulty. The law simply doesn't see most employment, by default, as an exchange of a set number of hours for a set amount of money. The enforceable parts of your contract can make the situation far more complicated than that. For examples, see: intellectual property and trade secrets, noncompetition, conflict of interest, overtime for salaried exempt employees, anti-moonlighting, accidentally representing your company on Twitter, and so on.
This stuff is very relevant to startups, as it's an easy way to torpedo yourself before you even pick a name for the company.