I don't know, but I think they need an additional license, which I believe is a contractor's license. They should have insurance too. However, I think many of them do small jobs on the side without that license. For example: adding an outlet to an exterior porch. They get referrals from family and friends. I'm not absolutely sure on all this though. I also don't know if there are union rules. Maybe they aren't allowed to do freelance work. Hopefully, someone here knows more.
I tried to research Local 3 Union rules, couldn't find the specifics in regards to side jobs.
However, from life experience I can't think of one electrician that DIDN'T do side work. It's cash money and off the books.