Because Vonnegut made an affirmative claim that this doesn't hurt children, and the comment was challenging that claim. I didn't notice anyone making a claim that it definitely does hurt children.
So no, the onus of evidence is on people claiming that rough language hurts children in some way. (Note we're not talking about abusive language here.)
> but, I don't think this is really what people want their kids reading […]
This is actually the initial claim, and the Vonneguts quote was a response to this claim.
Parents used not to care about what their children hear or read, until society told them it's bad parenting.
> don't damage children much