You've seen neglectful parenting then, which would exist irrespective of what material playgrounds are built from.
I suspect you either don’t have children or you are an extremely militant helicopter parent if you can tell us with a straight face you’ve met that bar.
This is not a high bar.
And yes, it's a full time job (at certain ages), and no it's hardly the primary responsibility, it's one of many.
And I think you misread; I said it was one of the primary responsibilities of a parent to keep toxic things out of your child's mouth.
Apparently you've never met an infant.
It is not a tenable position to suggest that children will unavoidably poison themselves, and that's just part of life...
Most parents will sit on the side and watch their kids. They can remove the tire material from their mouths, but they can't stop it from entering in the first place.
Also, having watched many a young child, this isn't normal behavior for children you leave alone to play on playgrounds, precisely for this reason.