This has little to do with the homeless and everything to do with a society that's shifted from seeking to facilitate positive things (e.g. the comfort of some random person on some random occasion) to one that wouldn't invent fire or sliced bread if it thought that doing so would be good for the wrong kind of people.

This is intensified in spaces administrated by government due to the incentives of government and the type of people who are best retained and fill out the org chart of such organizations and it is obvious because these spaces are most public but it's a thing everywhere, for example your hospital has security that could kick out "bad people" (whatever that means) but it still has a crappy waiting area not because they don't want to make it inviting for people who care about you to stick around lest they be there to raise a stink in the event you are mistreated.

There are comparable examples of this sort of "make things worse for people who are doing fine things" in all sorts of public and private contexts beyond just seating. I wish it was just the benches.