I had never read Twilight but knew a lot about the plot etc and had been involved in a lot of feminist analysis (how bad Twilight's themes are etc) when I first read a Twi fanfic. It was The Office and I really enjoyed it, and since then I've read loads of AU/AH Twi fics and yet continue to think very badly about the series. You're right in that the AU/AH fics have next to nothing to do with Twilight and that makes them perfect for this particular leap into the mainstream. For many of them, the only real similarity is the names of the characters and the fact that you can paste the actors heads onto the characters.

I'm opposed to publication of fic both for the reason you mention and, selfishly, because reading fic has been one of my favourite things to do for the last sixteen years and I'm concerned for what this means for fanfic as a genre. However, on the flip side I'm extremely glad to see that writing by women, for women - particularly of a pornographic-type nature - is making a splash in the publishing world. Mainstream publishing is extremely dismissive of non-white/male/straight/etc voices, and the internet is more of a level playing field. As unfortunate as it is that 50 Shades has been the public's first real introduction to fic, I do think some good things can come of fanfic moving into the public realm. Much like even though Twilight (the movies) contains horrible messages about women, if you look for a silver lining at least you can say that Hollywood knows that girls/women will actually go to the movies and spend $$ if you serve them up something not focused on reaching the male 18-30 demographic.