Why is it bad to make money off of something purchased from a professor?

After all, if one can't make money off of a thing purchased from a professor, there's no reason to buy said thing from a professor. Is it wrong for professors to make money from what they invent? In many cases, the school also makes money. Is that wrong?

Or, is it just wrong for middlemen to buy and resell? In other words, professors should only license to end users.