I don't know about the US but anecdotally it's been definitely the case for most of my work contracts in Brazil and Sweden for the past 17 years, I believe that

I don't know about the US but anecdotally it's been definitely the case for most of my work contracts in Brazil and Sweden for the past 17 years, I believe that only 2-3 startups I worked for didn't have a clause about moonlighting requiring pre-approval from management.