I have a Colt Cobra .38 that I bought from a police auction 30 years ago. It has been back out on the streets ever since. It has never returned to a life of crime.
Bottom line is that there is no obligation for the county to resell those firearms and there is no 2A infringement. Is it sad to see potentially valuable guns destroyed? Sure.
On the other hand they're doing the shooting public a favor destroying all of those Jennings, Bryco, and Phoenix guns.