I presume that parents take their kids to these playgrounds because there's nowhere else to take them. Judging from their state of repair, they serve poor neighborhoods with no tax base. If you live in a block of apartments with no play spaces, and these crappy playgrounds are the only open spaces around, then that's where you take your kids so they get fresh air and exercise. And the play equipment is probably not particularly scary to them compared to the crack house on the corner or the gangbangers on the stairs.
"Protecting children from frightening things" is a common desire, but it isn't practical and it isn't good for them. Children are little learning dynamos, and one of the things they'll learn if adults let them is to cope with being afraid. Ditto "confusing ideas." If there's one person I don't want to have to deal with, it's an adult who was raised on simplicities and certainties in this complex, uncertain world. He won't have had a chance to learn to think.
Besides, reactions are individual and cannot reliably be predicted. To assume "my reaction is valid; therefore, a reaction that differs much from mine is invalid" is to create a false dichotomy and to disrespect all outlooks but one's own. I was struck with the beauty of the olive oil and crumbs on my plate at lunch; the underwater sequence in Fantasia seems sinister to me; I often find clowns sad, but never scary. These are valid reactions, but I would not argue that someon who sees a mess on my plate, finds the animated sequence in question beautiful, and is frightened of clowns is "wrong."
Children, like everyone else, must be allowed to have their own reactions, and the adults in their lives must then respond as needed to those reactions. If you assume that the child will have a certain response, and pre-emptively act to prevent them from what you assume will be negative experiences, you never get to know the child and the child never gets to know the world he lives in.