It's possible that the flagging has come from user submitted reports. In that case if Google trust the reports (and they have enough data about users to know if reports are likely to be genuine) then they don't necessarily need to know any more details.

Alternatively it could be vague to restrict the possibility of bad actors circumventing the letter of the rules without adhering to the spirit of them, or even just protecting themselves from legal repercussions (perceived or real).

Your later point is the one that concerns me. Organizations like governments have issues where the spirit of the law is valued over the letter due to inertial restrictions over revising the law - when it comes to private corporations the ability to restructure rules remains unless it's explicitly surrendered. In these cases keeping the set of rules exposed to the public (and even demoing changes) can allow revisions to those rules to increase their accuracy.

And, when you get right down to it, any rule that isn't well structured will be exploited by bad actors, people looking to roll out malicious browser extensions have a strong motivation to try and discover those rules with a high level of accuracy by testing them - only the good actors remain uninformed.