Is this ethical? Depends on your point of view. Is it cheating? Not at all.
The academic integrity office has trouble condoning the system outright; one official claims there is "nothing inherently wrong" with it.
Technically, it qualifies as a solicitation violation in USC's Acceptable Use of Technology policy when students email their class's ListServ to advertise the notes. But who hasn't seen the occasional and irritating, "Sublease my house for winter break" email? Unless the university is going to start actually policing class ListServs for spam, we're doubtful the issue will be brought up there.
The real issue is that some professors don't like students profiting from what, in some cases, is merely other students' laziness. They don't want their classes to be so easy you can skip all semester, buy some notes and get an "A."
We understand that. But comparing it to cheating is ridiculous. If buying notes from another student is cheating, what about reading Sparknotes? What about study groups? Should students not be able to collaborate at all on studying efforts? Most teachers give out study guides themselves, but students still have to put in the time to memorize and understand the information.
We're all paying to be here. No one forces us to shell out the thousands of dollars it costs to enroll. If students choose to do the bare minimum to get their desired grades, that's their individual decision.
But they should understand that if they don't learn the material, they're not getting their money's worth. Even if they're paying for notes.