I disagree with your question in two ways (and mostly agree with the answer by Toshinou Kyouko). In fact both of them have already been discussed here before.
First, regarding the question of whether adding "legally" is a good idea:
The primary purpose of all community moderation policies is not to enforce any laws, including copyright law. Copyright claims are handled by Stack Exchange Inc. We have no part in this process. In fact, nearly every post here which contains an image is probably technically violating copyright law (though some may still qualify as fair use). If the rights-holders wanted those removed, they could go through Stack Exchange's designated agent and follow the standard DMCA protocols. But those sorts of minor copyright violations aren't the target of our community moderation.
Rather, our goal in explicitly prohibiting questions "requesting illegal or copyrighted" works (as per the help center) is because we're trying to create a reasonably respectful and professional Q&A site on this topic, and explicitly promoting pirated content on this site runs against that interest. If people really want to steal products from creators, they can probably find many ways to do so, but we don't want to be helping them with that.
There are 3 kinds of common incoming requests for anime/manga. We sometimes get ones asking explicitly for legal sources. These are not in any way in conflict with the above. We also occasionally get ones which are explicitly asking for unlicensed sources. This second category is explicitly off-topic according to the help center, and we have an off-topic reason for these questions. The third kind are questions which are not explicit with regards to this, and how to deal with them is also not as clear.
A previous question on this third class of questions already exists: Should questions asking for translated anime/manga without specification be closed?. The consensus there is to give users the benefit of the doubt and assume they were trying to ask a question in the first category, but also to explicitly edit this into their question and preferably leave a comment alerting the user of this policy in case they are not aware. If you disagree with this, you're welcome to add a differing opinion there, but so far my answer seems to have a pretty strong consensus.
Second, regarding whether these are in fact good questions:
I disagree with you that these are uniformly bad questions, but I'm sympathetic to the viewpoint that many of them do not show much research effort. Others are rather specialized and will only be useful for a small number of people. And this information can change, making such questions similar (but not identical) to the "unannounced future events" questions that we do close (e.g. "Will anime X get a second season?").
None of these are really fatal flaws though. It's possible for a question asking about releases to be legitimately challenging to answer. One needs to track down various press releases (not always in English) and follow through, which could even involve contacting the customer service department of the licensing company to see if there have been any updates. This can actually be surprisingly challenging in some cases. Regarding small audiences, many people import anime releases (since they're only officially released in a small number of countries), which tends to make these questions apply to a larger audience than one might otherwise think. And regarding being like the "future events" questions, these are different in that they're objectively answerable at the moment. The answers might later become obsolete, but we can deal with that just like with any other question where that is a problem: edit and/or post a new answer.
When you see a question which has any or all of these problems, you should do (at least) one of two things. If you can improve the question, edit it to fix the problems. If the question demonstrates a lack of effort on the part of the asker, downvote it.
We've also discussed this topic before (see How should we deal with questions asking for where to buy, watch, anime or manga media or such related merchandise?), and the (again strong) consensus was to allow these questions. Since then, I've not seen us be overrun by such questions; there have been maybe a couple dozen. You're welcome to post your opinions there on this if you disagree, but I'm not seeing any evidence for your claim that we're going to be flooded by these.