It doesn't make any sense to me either, but there are some het/gen ficcers that don't want to be archived with slash, just as there are slash writers who do not want to be archived with het/gen ficcers. And, yeah, I have first-hand experience with both in Buffy fandom.

Most people don't think that way, but there's always a subgroup that does. Thankfully,the Internet is a big enough place that they can go to their own corners and not bother anyone else. Usually.