It’s really not, at least not in my state. I don’t really have a “contract” per se, just an offer letter and a set of company policies, many of which are legally unenforceable. I know multiple people at my company that have second jobs (i.e. a rental property, uber/doordash, etc), they just do them outside of normal work hours and certainly don’t share equipment between the various jobs.

And for references, I would never list my current employer anyway, because then they’d know I’m looking for a job. I might list an HR contact to verify employment and job title (if they need that, which is rare), but I’m much more likely to list someone unconnected to my current job as a reference. Most jobs I’ve applied to haven’t bothered asking for references, and my company certainly doesn’t bother with it either, either you get hired based on your own merits in the interview process, or you don’t. We do a background check once we send an offer letter to make sure you’re not a serial killer or something, but that’s as far as we go.