Possibly to keep it from harm, such as the guy didn’t have a fenced yard and didn’t want it slipping out the door into the streets or some such thing.
But harm kicked the door down and shot it anyway so it didn’t do much good.
“Why it was tied up” is hardly the big issue, though.
Or possibly to prevent something like this happening.
Pitbulls Euthanized After Attacking, Killing Thomasville Search & Rescue Border Collie
Even after the attack dogs owner had been fined in March.