I asked another dog owner and they said it was probably to ensure that people watch their dogs. Twice I have seen owners completely ignore their dogs so that other people have to ask around when they see a dog poop and no one coming to pick it up for a long time. These dogs could get hurt or choke on something while their owners are completely absorbed talking on the phone and/or reading or playing on their devices. I saw one guy with what looked like homework spread out on a bench and earbuds. His dog was apparently shy and wandering around aimlessly, and came up to him several times and was completely ignored. Since I knew the other humans and which dogs were theirs, I felt bad for his pup. My suspicion was confirmed when the guy finally packed up, called and leashed his dog and left. Can't say for sure why they don't completely enclose, but it does encourage people not to use it like a free doggy daycare, but with no staff to keep dogs safe and encourage the shy ones to socialize.
Gotcha, thanks Alice for the info! I hope the park would reconsider gating the park and have signs on general expected rules for people who bring their dogs in.
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