It does look as though zoos were willing to 'dump'(?) their surplus onto this place when it suited. There is nothing new in this as it has always happened.
Zoos historically have been about the saving of the species, and not necessarily the individual - sometimes at the expense of the individual. Modern conservation requirements still require this.
The difference now of course is that some zoos have positioned themselves as 'rescue centres', most often for bears from foreign countries, or lions from war-zones, or laboratory monkeys. These imports are very often of high box-office value, even if the animals themselves sometimes dont live very long and contribute nothing to conservation, and take up space which could be used for conservation-breeding programmes.
Maybe old, small-cats from England, mixed with a dose of 'dodgy' origins are not as appealing and media worthy?
Perhaps it is best for the individuals, that they are put down, humanely and quickly, rather than spend the rest of their lives in some other place like this, contributing nothing until they eventually have to be euthanased in the end. Few animals, domestic or wild, die peacefully in their beds - for them it is not a matter of when euthanasia is done, it is how it is done that matters.