Four feet below standards is not meeting the standards. And if four feet higher still allows a tiger to get out, obviously those were also not safe enough standards.
Unless you think the point of caging a wild animal isn’t to keep it caged.
Zoos are a trade-off between keeping to animal in a specific area, away from being able to harm the public.
AND
The public, meanwhile, want to see the animal as closely as possible. Some want to do from the ‘safety’ of their car, others want to touch the animal, even at the risk of minor injury.
All the definitions are always in a certain amount of flux, including the definition of ‘safely’.
No one want to ‘see’ the zoo animal at 100% foolproof zero risk, because that means the 50 foot solid walls I’m talking about may not be enough.
There aren’t “standards” ...
There are “recommendations”.