Just because something is popular among kids doesn't means it's marketed to kids. Kids are into forbidden fruit, they're also into annoying their parents. By 10 kids should be in bed, my school night enforced bedtime was 9 until my age hit double digits, then it got bumped to 10 until mom decided I was old enough to screw up my own life (15). So you see, when parents have rules and enforce them their kids won't be seeing South Park, when parents don't have rules or enforce them that's not Parker's and Stone's fault.
"when parents don't have rules or enforce them that's not Parker's and Stone's fault."
Excellent point. But in fairness, I'm sure there was nothing on tv as coarse as South Park when you were a kid.