OK, so I asked the local FB group, as people suggested, and here’s the deal on the inaccessible wheelchair-accessible swing.
It was built by parents of a disabled boy. They got a grant and some donations from local students to build it. So the swing is for their son to use, and they don’t mind other disabled kids using it in their presence.
People are very proprietary.
This is a very sick boy with a rare brain disorder, so I don’t want to criticize the parents.
Clarissa's Blog 
The fence seems completely weird. Only explanation I can think of is there might possibly be a liability issue if a child were injured. I wonder if the local government might have to pay higher insurance premiums for the playground if the special swing was not fenced off. Not likely, but possible.
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But it’s in a public park. So it should be available to all kids in wheelchairs who come to the park. Otherwise, really, what’s the point of having the swing in a public park if it’s just for one child?
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I agree. It’s a large structure that takes up a lot of space and it’s weird that it’s not accessible to anybody.
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