I am a college student who is currently enrolled in a class where the professor posts lecture videos online with ads enabled. I was wondering if this is legal; the professor is being paid by the university to lecture, but is also making money from ads on his lecture videos.
Legal? Probably yes, depending on university policy.
Ethical? Universities and politics nowadays encourage academics to be business-savvy and profit-oriented (I guess, as long as the uni gets a cut). So, while the described constellation indeed carries a "smell", it may be perfectly in line what society wants academics to do.
If the institution provides the server and other necessary resources to host videos, then the students are essentially paying for this through their tuition and shouldn't have to put up with ads.
However, if the professor provides the recording equipment etc., and has to arrange for the video hosting, etc. without this being provided by the institution, then I don't see anything wrong with this. It might even be that the video hosting site imposes the ads and the professor doesn't profit from them at all.