Because there's a control scheme built with the Book of Vile Darkness (read Bible, not the D&D item) but not any D&D books. This on it's own wouldn't likely be enough to see D&D books burned but because there's a control scheme in play that feels threatened by anything that brings people joy 'X said something bad because D&D' is a sufficient justification in their eyes.
Saying shitty things that re-enforce common presumptions is lucrative.
I had an idea of wanting to start a cult that featured doctrine that I agreed with. I wonder if every moral philosophy enthusiast goes through a why can't I start a nice cult phase. We can't because the cult doctrine is the same, consistent hate based bullshit. It's your woes are caused by that batch of undesirables over there, and God should smite them every time.
If you want to plant community gardens and feed the hungry and enrich the impoverished and give voice to the silent, you'll lose all the people who want to yell at kids to get off their lawn.
Whereas the truth is, they're in the same place. The same place that bigot will be when he dies and the same place we all will be when we die.
And it's too bad he won't have any awareness of it after it happens because the schadenfreude of him finding out he's not going to have an everlasting afterlife would be nice to enjoy.