I don't think it's a problem. The whole point of subtext is that it's inferred from the existing text. If someone tells me that she's a lesbian, I can possibly infer from her behavior that she slides on the (outdated) Kinsey scale.

Likewise, assuming that a label on fanfic can excise any imaginative treatment of the story on the behalf of readers doesn't give some readers enough credit.

In regards to the show, since slash writers draw their inspiration from the text, writing explicitly queer stories about it does not mean that the subtextual elements are denied in some way. It's an expansion rather than a reading that shouts that it is against the grain of the narrative, and since fanfiction takes place in a non-capitalist arena, it's a condemnation of market elements present in commercial texts that prevent explicit queer elements from showing rather than a statement that "this doesn't exist on the show."