Sweeping assignment of all copyright—including for things done off-hours—to your employer are common features of employment contracts in the US, which means no

Sweeping assignment of all copyright—including for things done off-hours—to your employer are common features of employment contracts in the US, which means no open source work without requesting an exception. This is true even in many startups. Absence of that kind of provision is seen as risky by investors or acquiring companies, who fear current or past employees will try to claim parts of the company's IP as their own. Most are chill about adding exceptions (they don't actually want to claim copyright to that children's book you're writing in your free time) though. Still, sucks and probably ought to be illegal.