Is it ethically acceptable for university book stores to market homework help websites?
It’s helpful to approach the question starting from the following general guiding principle:
It is not acceptable for a university bookstore to engage in misleading marketing practices of any sort.
If the way the subscription link is presented creates an appearance of endorsement by an instructor, and such endorsement was not given, then the marketing is misleading. If this was done intentionally, then that is unethical.
If the link suggests the service is endorsed by the university but not by the instructor (or is simply offered as an optional purchase without the implication of any endorsement), it seems hard to argue that it is unethical, or at least hard to argue it is more unethical than any other link to a product that anyone puts on their website. It’s mainly creating the appearance of an endorsement by a specific instructor who didn’t give their approval that would be problematic.
I cannot be sure from your description that I would agree with your assessment that the way the information is presented creates such a misleading impression. But I might, or perhaps I’d agree that it’s sufficiently ambiguous as to risk misleading at least some students.