Colleen the shelter certainly didn't do it because they didn't care about your pup's health. It's standard procedure for any rescue or shelter to only release altered animals in order to prevent further unwanted litters. You would be surprised just how irresponsible some people can be with keeping their intact animals from accidentally breeding.
That being said, a lot of the new medical and research material coming out is pointing more in the direction of delaying altering until the pet is fully grown and has benefited from the presence of the hormones. This is especially the case in large breed dogs. That should always be balance with your own personal ability to maintain an intact animal, and some times behavioural issues may be a factor too (intact males marking or becoming aggressive or territorial) that an average pet owner may be unable to deal with.
It's all a balance and in most part a personal preference. The best thing you can do is read up on the subject and form your own educated opinion.