Apple tries to justify their cut saying that they're promoting your app which is leading to some of your sales. That's total BS - 99% of my sales came from when MacWorld ran an article on an app I was selling. If Apple really did anything to drive my sales, MacWorld wouldn't have had such a major impact.
I don't think the cut is for promotion. Its for selling your products. Just like a real store selling your boxed software on CDs which will ask for far more than just 30%. Now could Apple ask for 20 or maybe even just 10% since its "just" software and no physical media? Is it fair? I don't know, but you have the choice of hosting it on your own and doing all the business stuff by yourself if this cut seems unfair.