Simple answer. Most of us (and most of the world) thinks At-Will employment is barbaric.
It is entirely reasonable to require some substantive effect to warrent termination, even if that substantive effect is not directly the teacher’s fault. Her having an onlyfans account, not grounds for firing. Her onlyfans account passed around by students? Grounds for termination.
There’s a (not so new) trend in the US for companies to crack down on side gigs. Yes, sex work is a politically charged side-gig, but we shouldn’t ever be supporting a company’s right to fire people having side-gigs without a very good reason. So long as your side-gig never encroaches into your day job in any real (not hypothetical) way, there really isn’t a good reason.
There are valid criticisms to UBI (usually specific to each implementation), but “lazy workers” will never be one of them.