Is it legal / ethical for a professor to monetize course content videos? 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.