I think that's all these schemes are really, I don't think people do it to get a realistic idea - if they wanted that they'd arrange a work experience placement. People pay quite a lot of money to go behind the scenes for the day.
"Keeper for a day" is just a name. I don't see any problem with it, if you're paying a lot of money you want a good experience, not to spend the whole day deyolking day old chicks. It raises revenue for the zoo and as you said is a fantastic opportunity to get up close.