If the dog nips once does it deserve to be put down

A lot of shelters but there dogs are a probation like period to see if it is adoptable. One excerise they do is feed the dog out of a bowl and have a fake hand try to grab the bowl. Now if the dog nips or growls or does anything aggressive they sometime just but the dog to sleep. i think that this is just plain out wrong. how could someone just do that. What i think shelters should do is if the dog does show to be aggressive then they should work with the dog for a period of time to get him out of that. Just because someone abused a dog to make it bite and then ends up being aggresive when in fact it could of been the nicest dog ever is not the dogs fault and should not be treated as being the dogs fault. Why cant people just work with a dog to help it so it can be adopted rather then just give up on the dog. Put it in a human point of few. if you are a foster parent who adores a kid but he was brought up in a bad home such as his parents doing drugs and his father  hitting people  and he has a bad temper and hit peolple also because you care for this child you seek professional help so he can change the way he is right. Well thats how it should go with a dog. Dogs have feelings to dont forget that