More Words, Deeper Hole

I would suggest that there's no restriction against their being members (though I'm not in SFWA and wouldn't know) based on the fact that writing for comics is essentially done in script format. Unless you write the Marvel method, which is altogether different.

But personally, having worked in the medium (however briefly), if I had the choice, I'd want to be part of an organization that doesn't exclude the artists. No comic book gets made without artists.