Stewart has said on her daytime show, more than once, that she doesn’t see all the taped interactions between the candidates on the The Apprentice after the show is done. She bases her decisions on what she’s told and what she sees and hears in the conference room (I guess because that’s what she’d have to do IRL, where her workers aren’t followed by camera crews all day). So she probably didn’t fire Jim because she didn’t know exactly how badly he was acting in the store, and Jennifer didn’t bother to tell her.
(Stewart also seemed bothered by the fact that Jennifer is a public defender and yet was doing a crap job of defending herself. True, a courtroom lawyer should have a better grasp of how to arrange and present her case; on the other hand, in court it’s not a lawyer’s job to keep order, it’s the judge’s. Jennifer might do perfectly well in the more structured environment of a courtroom while still being a bad business manager.)
As for why Jim is such an ass, he may well have figured out that’s the way to reality-TV fame. Does anyone remember any of Trump’s first batch of apprentices except that show’s hate object, Omarosa?