It’s not as if the man was going to “PRISON” for a year or more. Even then, they could just put the dog in the pound and let him be adopted out. To think he was killed while the man was just put in jail (not prison) is beyond reasonable. I don’t see any good reason to kill the dog, unless after so long no one adopted him (standard OP for a shelter).
So you have this old guy, who cannot restrain his dog. The dog becomes a ward of the state, and everyone else pays for his care. (And the probability of the dog being adopted-out is probably around 10-15%). So the dog gets euthanized anyway, but only after being mistreated even further.