He usually doesn't like to pirate from small companies or professors who are trying to make a living by selling books, but from millionaires & plenty of mega corps which already have loads of money, he feels like it's the right move to pirate
Even if you paid for everything, most likely these professors are barely going to get anything out of it. They get into shitty contracts with big publishers. So unless they publish stuff on their own, you're not really helping them by buying their books. Oftentimes, you can just email the profs and they'll send you a free pdf of their stuff because they just want to get their ideas out there and don't care too much about the money.