Here's the trick. As anyone who has run movies for a convention since the 1980s knows, you can't do a public performance of a movie sold for private use. That violates the "public performance" right and you pay a separate fee for public performance. This is true for DVDs sold for rental. The license terms are different than DVDs sold to individuals for personal use.
Now that Netflix is very successful, studios want to (a) protect the pre-existing lines of revenue from sale and video on demand, and (b) get a taste of Netflix's revenue above and beyond what they get from the initial licensing because, hey, why not. You could answer "because you stupid greedy idiots you are screwing up your market and you will make less money," but these guys have a long history of not understanding that.