The biggest problem here is not the support cost, but that offering this kind of information gives people insight into how the algorithm works so they can better game it.

In search, email, etc there's continual competition between people trying to trick the span detection algorithm and people trying to extend the algorithm to counter them. Secrecy is a critical tool on both sides of this fight. Force transparency on the algorithm maintainers and it's output will get much worse.

Keeping the algorithm secret has downsides, but is worth it (for society at large) on balance.

(Disclosure: I work for Google, on unrelated things, and don't know anything Google-specific here.)