There are a number of reasons why some dog owners tend to give their pet up for adoption which include bad dog behaviour, lack of funds to take care of the dog or for other reasons.
Bad dog behaviour is never a reason to give a dog up to a shelter, as a dog can easily be house trained and turn into a loving pet.
Some owners do get their dogs taken from them for some reasons which could also include cruelty, I do not see a reason to be cruel to a pet. It's uncalled for and just should never happen.
By the way if you are looking for a pet to adopt you can use the following shelters:
I am sure that there are many more, but those are the ones I can think about off head.