Justine Larbalestier demolishes this argument thoroughly:
I’m not going to link to where I saw this particular bizarre notion. Mostly because it’s not something that’s found in one place. I’ve come across the same sentiment in various locations offline and on- over the last ten or so years. So it’s kind of irrelevant who said it most recently.
But here’s [the] gist of the argument:
YA writers only do it for the money. They don’t care about the effect their [insert negative adjective] work has on children only about making money.
Most writers I know would agree that 'doing it for the money' isn't really a prime motivator. If you really want to make money, become an accountant or something. This is also Justine's experience:
I’m fascinated that this argument gets made at all ever. I don’t know a single writer who became a writer to make money. Everyone I know is a writer because they can’t not be a writer. It’s a compulsion. A vocation. Something they do whether they’re paid for it or not. This is true across genres.
Have a read through her own experiences as a YA author and why she writes.