Because that's the thing about Scooby-Doo: The bad guys in every episode aren't monsters, they're liars.
I can't imagine how scandalized those critics who were relieved to have something that was mild enough to not excite their kids would've been if they'd stopped for a second and realized what was actually going on. The very first rule of Scooby-Doo, the single premise that sits at the heart of their adventures, is that the world is full of grown-ups who lie to kids, and that it's up to those kids to figure out what those lies are and call them on it, even if there are other adults who believe those lies with every fiber of their being. And the way that you win isn't through supernatural powers, or even through fighting. The way that you win is by doing the most dangerous thing that any person being lied to by someone in power can do: You think.
Tim Minchin fans may recall him mentioning Scooby Doo in a similar light in his beat poem Storm, and it's been brought up on Less Wrong before.
When viewed in this light, Scooby Doo really is like an elementary version of Methods of Rationality.