The probation is known in legal circles as "covering your ass." My wife found out about this the hard way - an unsubstantiated (read: she didn't do it, but we know who made up the rumor) rumor made it's way to her boss. Bammo, she goes on "probation" for two weeks, with strict rules supposedly to shape her behavior. She pulls it off without a hitch. They bring up a whole host of "new" things that were not mentioned before as further excuse to fire her ass, so they don't get slapped with unemployment claims.
This happened about two years ago, and I wish I could have talked her into suing the bastards. They didn't really prove themselves in their claims, but she was too much of a wimp.