The problem is two-fold. First, there is still some precedent for some dependent tags (calling them Meta tags is sometimes a misnomer, let's not overload terms here) to exist to some degree, the problem being that the degree of their existence has yet to be established. We find that being able to divide, say, zerg questions from the full body of StarCraft 2 questions is helpful. But it hasn't been fully established the conditions under which they should be applied overall. While this remains unestablished, people don't have any solid guidelines with which to apply dependent tags, or to not apply them. You're correct that there is no complete policy on their application or non-use.
The more prevalent problem is that while we established that many dependent tags are bad, we haven't cleaned them all up. There's a lot of tags to clean up - Jeff, I, and others have been targetting some of the larger and more problematic ones, but we haven't truly finished the job. As we know, "precedent" does a lot to convince people to continue their usage. And we have a large history of "precedent".
I guess the ultimate takeaway here is, we should not be surprised that we are still running into these. There's a lot of backlog on this and we're probably not even a quarter through all of the tags. What should be done is to take action - either start a discussion if the tag is borderline, or just take action and demolish the tag if it's unquestionably toxic. agent86 did some fine jobs by bringing up tag batches that need cleanup - the more we can clear, the more we get done.