They are allowed to be kept and bred in captivity, just not in the United States and releasing them back into captivity would not be a good idea since captive raised garters would not survive long in the wild. There is also the problem of their shrinking habitat and main food source. If more were released into the wild, I think it would hasten their demise. Of course the way way things are going for them, I am not sure if they will be around in 50 years.
Jason