According to everything I've ever read from reputable people, they do spay and neuter them, and they do have vets see them. BUT have you ever lived around feral, or basically feral, cats? They HIDE. They are sneaky and you can't always find them. Especially cats soon to become mamas. So you're not going to always know where they are.
And if people continue to dump cats there...you can see that there are going to be cats that get around the system.
Our first cats were basically feral, they came to us after their owner (old lady with 18,000 indoor/outdoor cats) moved or died, and we fed them chicken pot pie on the porch for weeks before they were OK with us being outside when they were there. Then one of them disappeared and we noticed the one that kept coming back was getting bigger. The other was a boy but we'd never been allowed to get close enough to see that fact. The mama had her kittens (took us forever to find where she hid with them!) and the kittens let us handle them, but only my brother was ever able to hold the mama for more than a minute (she was "his"). After that she was spayed and we gave all but one of the cats away (one became a winery/barn cat, another went to friends, and, well, I was only 9, I don't remember who got the other) and kept the other (that one became "mine").
My point is, with a semi-feral cat, you're not always the boss, and especially if the only time you can try to snag them is in the dark (after the park closes, before it opens), you're REALLY not the boss!
But I think they do all they CAN do for them.
"Wow . . . I've had a DL annual pass since 1989 and I've never seen a mouse or a rat at DL."
Look in the bushes/landscaping around Matterhorn and/or NOS train station. tiny mice. So stinkin' cute.