Please keep your dogs under control!

I took the ride on mower from the caravan park to its winter storage site today.
When walking back, I saw three deer and this is one of them. They were just wandering off when “whoosh”, two springer spaniels appeared and started chasing the deer.
Two successfully jumped over a fence but the third one got a bit entangled. Fortunately, she managed to break free and I’m pretty sure she was okay as I saw them running away into the distance.
The dog owner appeared. He had been trying to call the dogs but of course they ignored him.
Just why didn’t he have his dogs on leads when he must have known there might be deer about!