My take on the issue is:
We (MSE, and SE in general) have a policy that whenever a question gets asked here, it belongs here in a sense, and it is subject to the standards we have here. This sort of ownership is behind the principle of frowning upon self-deletion of useful content, for example.
There is some controversy regarding hint-like answers (this has been discussed relatively recently, and comes back from times to times), but it seems that it is widely accepted (here and elsewhere, corroborated by how the system of Q&A is itself envisioned) that detailed answers should always be on-topic (this is not saying that they are necessarily useful; more on this later). The point being: questions should be answered. This is the backbone of the concept of a Q&A site.
But... if we take that literally, to the extreme, quality (and even ethics) suffers. In order to prevent this, we have some barriers to prevent people who come ill-intentioned and/or low-quality questions. This puts some restriction to the previous "point", it now being: genuine, on-topic questions should be answered. What, exactly, those barriers should be is a little controversial. But I think it is not controversial to say that your question passes through these barriers.
Being a detailed answer to an on-topic question, that answer has its place here as well.$^{(1)}$ As far as how the system works go, there seems to be no issue at all.
The problem seems to be the concern regarding other people, not involved with the question or the answer, to grab an answer for their own use while skipping important processes of learning, circumventing the "genuine" part of the process on their end.
This is a valid concern, but one that is ultimately impossible to have anything practical and enforceable upon in my opinion. The only solutions which I can think of that would work on a large scale would be things like discouraging detailed answers to textbook problems, banning textbook problems etc, which seem to be all problematic.
One solution that could work in general and that may be fair is to let an author explicitly say that they do not want their questions in this site, and that the community respect this. But once one agrees to have their questions in this site, those questions (and their answers) should be addressed as per the procedures of the site itself.
Now, having addressed the general issue in those last three paragraphs, one is of course still free to think that some answers being complete, detailed solutions in certain cases are not useful. In those cases (and the particular situation alluded to by OP could be one of those), I think we can rely in the voting system. If you see an answer to a question which you think ends up being not useful due to the whole context of the question (or for some other reason), then downvote it. This is really not specific to this situation: recall that when hovering over a downvote of an answer, it says "This answer is not useful"!
$^{(1)}$ The community can discuss possible barriers regarding complete answers to textbook problems, but I personally think that this is a bad idea to be done systematically.
