Without more information, it is hard to make any ethical judgement. The instructor might have a say in what appears, for example.
But the more serious issue I see is that, if such things are published and available to some students, even just the ones that are more diligent in taking shortcuts than in learning, then it might actually be essential to make them publicly available to all. This "levels the playing field", so to speak, so that everyone has easy access to the same materials.
The instructor should, somehow, be made aware of such things so that the course can be designed in such a way that they don't interfere.
But, if they exist, and can be found, then they should be open to everyone. It is much more problematic if they exist but are only known to a few and unknown to the instructor.
And, knowing about this, the instructor can respond, perhaps in the syllabus. Anything from "This is a valuable/essential resource" to "You will be wasting your money with a subscription". The professor could also, perhaps, buy a subscription and make credentials open to the entire class. It would be a pretty cheap TA, I think.
And the faculty can, if necessary, respond as a whole, though I doubt that banning such a practice will make education better. It is the existence of these resources that have issues, not the fact that they are known to all.
I would, of course, prefer to be the sole source of hints to my students, so that I can give minimal hints (letting them have "insights") and also assure that a question/answer from/to any student is seen by all (if a question is asked by one, it is probably one others have as well). But I have no way to guarantee that. I did, for several years, run a mailing list for each course that had those desirable properties, making other things moot.