Questions are very often reused, even if the order might be changed or some new questions might be mixed with the old. So having previously asked questions that are likely to be asked again, and especially having the answers, is almost certainly an unapproved resource.
In general, you’re supposed to 100% do your own work and thinking and cite all resources that you draw from at an American university. In some ways it’s complicated and a large number of honor code violations (cheating) are committed by students who honestly don’t realize they are violating the honor code.
If you really want to get solid information on whether this is likely an honor code violation, you could ask at Academia.SE. I think it’s completely reasonable that the university they are attending considers it an honor code violation and they know it, which is why they can actually get paid for the material (if it were licit everyone would have it and share it) and why they are careful and concerned about their business.
From the UCLA code of conduct:
102.01e: Facilitating Academic Dishonesty
Facilitating academic dishonesty includes, but is not limited to, knowingly helping another student commit an act of academic dishonesty or publishing assignments, exams or solutions without permission of the instructor.
University of Southern California engineering school honor code:
Any use of external assistance during an examination shall be considered academically dishonest unless expressly permitted by the instructor.
Stanford honor code:
Examples of conduct that have been regarded as being in violation of the Honor Code include:
- Giving or receiving aid on an academic assignment under circumstances
in which a reasonable person should have known that such aid was not
permitted
Virginia Tech honor code:
Examples of cheating include, but are not limited to:
II. A. 1. Acquiring answers from any unauthorized source in completing any examination. For examinations, this includes looking at another student's exam, taking answers from another student’s exam paper, use of textbook/study sheet/calculator during an exam for which those materials are not allowed, working with another student on a project that is to be completed individually, copying solutions from an online source or solutions manual, getting answers from students who have previously taken the examination, or using external aids (e.g., books, notes, calculators, electronic devices, conversation with others) that have not been specifically designated as allowed by the instructor.