Please read this link and I encourage you to post comments if you are horrified as well.
I have to say I'm not 'horrified' by this- far from it as such behaviour/interactions between Chimpanzees(and other primates) and the more moronic sections of the public in Zoos have been going on since time immemorial. Read Belle Benchley's excellent book 'My Friends the Apes' for accounts of the behaviour of US servicemen killing time at the San Diego Zoo and teasing the chimpanzee resident pair in the 1930's era, and also how the Chimpanzees cleverly retaliated. What you describe here is just another example of Visitors 'winding up' the chimpanzees, it happens in zoos everywhere, and on a very regular basis, so much so it can almost form part of the Chimps' daily routine.
The fact the staff hardly bothered to react or caution the human offenders in this incident means it probably happens very commonly at this Zoo too, perhaps an almost daily event and that they(like me) do not view it as serious. Even throwing a cup back is hardly 'abuse' -the chimpanzees choose to react the way they do, they could move away, turn their backs, or go indoors but they don't, they are simply stimulated to react with a display of perfectly natural hostile behaviour as if to a group of rival chimpanzees arriving on the border of their territory, a component of their natural life which is missing in captivity. So you could actually look at it as a plus for them really as far as their behavioural repertoire is concerned. Any sign of diarhoea, or fear grimacing on the Chimpanzees part?-I doubt it as that would indicate real stress. What they are demonstrating is normal confident hostile behaviour to would-be intruders to 'their' territory. I wonder how many times a week this happens when you aren't there to witness it. Routinely, I should think. And from the chimps' point of view, seeing the people finally leave the area after their excited displays reinforces that they have, yet again, won the dispute with the rival 'clan' and driven them away.
I also think you are taking an unrealistically sentimental approach to this issue and confusing a commonplace incident of 'teasing' with real animal 'abuse'- starving animals, keeping them in unsuitable surroundings or small cages, physical mistreatment. I'm sorry but I don't see this as belonging in that league at all. It shows the visitors up as stupid ******, but that's about all. Comparing it with terrorising elderly/senior citizens(human) is also erroneous as these are Chimpanzees, not people- however closely related we may be. The main human perpetrator can't be really be seen as an all powerful aggressor either- if you consider the adult male Chimpanzee, however old he might be, is still several times more powerful and has the potential to seriously disfigure, and quite possibly kill him if given the chance.
From the inflammatory tone of your headline to this thread and your stated membership issues and other unstated complications between you and the Zoo management, combined with the invitation for others to join in the critisism, I'm thinking you now have a personal grudge against the Zoo-but no Zoo would seriously allow visitors to genuinely 'abuse' its animals,or cause them serious harm, its not in their interests to do so.
