Why? everything that is held by force you have to pay storage for, as it costs money to do so.
Just borrow the money, or better still talk to the vet and try and work something out.
And everyone has money to pay immediately with, do you not think it is unfair to charge a high storage fee like that? I doubt it would cost the owners anything like £20 quid a day to look after a cat. You cannot just chose to return something and charge a storage because the person cannot pay then at least not without good reason.
And frankly a vet that is 'inhumane' to animals (maybe a little strong but I believe they should be doing what is best for animals and not acting unreasonably: unreasonable behaviour being key to this statement) should not be allowed to be a vet, assuming the vet has acted unreasonably here, animals are not just property they are a living creature and may be valued like a family member to their owners.
Of course owners are also liable to any vet bills etc related to them but extortionate storage fees? Now if there has been a history of non-payment from said owners then that may add complications.