It's sort of like going to a fast food place. They have the option where I can buy a hamburger, fries, or a drink... or maybe get it in an all for 1 package at a cheaper price than buying all 3. Usually someone gets the all for 1 package but sometimes people just want a hamburger or just the fries. If they forced everyone to buy all of it, then they would just buy nothing.
In the internet world, instead of choosing to buy nothing, there are a lot of people who will just copy it from a newsgroup instead of paynig for a bunch of stuff someone wouldn't be able to use.