You would be passing someone else's ideas off as your own. The same goes for posting your notes on the Internet without the instructor's permission. You would not have known that information without going to that lecture. The lecture was not the product of your labor, so just because you copied the words down on paper does not mean you can claim it as your own. The third point relates to companies and individuals outside the university environment.
Students have a right to take lecture notes for their own use.
It's like buying a compact disc and making a cassette copy for your walkman. It's for your own personal use. If you were to take that CD and make 100 copies to sell on the street, you would be sued for copyright violation. Buying the CD gives you the right to play the music as much as you want. It might even inspire you to make your own music.