There are two issues with university notes and plagiarism and they are worthwhile distinguishing.
Firstly, a uni student goes to a lecture, takes word-for-word notes and posts them to BOS. That is both plagiarism (it is not the student's own work) and a breach of the Copyright Act (the "word-for-word" aspect of the notes).
Secondly, a student uses content from BOS in submitted work, without acknowledging the source. That's plagiarism.
I wouldn't let fear of the second class of plagiarism stop you extending into aiding people at university. All instructional materials run this risk. In that way the media attacks on BOS are most unfair -- textbooks don't get criticised for encouraging plagiarism, despite being the source of most of it.
Where I would be cautious is with the deeper depth of knowledge required for university. One of the reasons BOS is successful is that there are many people that can help -- from professionals in a field like me, to students that did the subject in previous years, to fellow struggling students. But there isn't the same number of people attached to BOS that know about "CPU scheduling techniques for non-uniform memory access", or many of the other arcana university teaches students.
I also wonder if using BOS would be a good first tactic for uni students struggling with a particular topic. You are paying massively to be at uni. Take advantage of what you have purchased and attend the consulting session for your subject to get your understanding ironed out one-on-one with an expert. If there is no consulting session with staff then that would be interesting to hear about on BOS. "Ease of access to staff" is one of those things vital to the quality of your education which doesn't appear in any university course guide.