Not sure what country the person works in. In addition it could be a situation of a small town / tech backwater and word getting around, regardless of the legal

Not sure what country the person works in. In addition it could be a situation of a small town / tech backwater and word getting around, regardless of the legal question, so quitting out of contract could get you blacklisted. Ultimately though a person could not show up for any reason at all (e.g. hit by the proverbial bus) and you need to plan for that. Sounds like a company that just flies by the seat of its pants so they probably have that 3-month clause for that reason.