I wonder if some of these celebs who turn on the charm for the camera but then return to unpleasant afterwards genuinely don't see anything untoward by it.
Maybe they just see the niceness as part of their job, so once they've stopped doing the job, there's no need to keep it up - just like actors playing characters who hate each other, having a blazing argument on camera, but then going off together for a coffee and a friendly chat once it's a wrap; or, for that matter, a supermarket checkout assistant not wanting to keep scanning goods indefinitely once their shift has finished?!
I'm not saying I like their attitudes in the least, but I do think that some of them would find it very hard to grasp the issue when people comment that they're not like their on-screen persona at all. It still doesn't stop anybody from being a nice, civilised human being by default....