When I was an hourly employee in the eighties and nineties, I was told that many U.S. companies had a clause indicating that they could fire an individual for ANY reason before a probationary period had elapsed. I don't know whether certain reasons were considered illegal, but I should think that they would be. But the point is that the companies apparently didn't have to specify a reason.
If my job included such a probation, I would not divulge until after I had completed it.