They take the 30% out so Apple can "run" the App Store. I'm sure they could keep it up and running fine for less.
I suppose your right in a way, but consider how many apps Apple is taking 30% from, and how much those apps are selling.
Do the math. If there's $5,000,000 being made, and Apple gets 30%, it's only $1,500,000. There's a lot of file in the store, and Apple probably makes a small profit for themselves.