An analogy for this in the real world would be tips for waiters. As a foreigner, I'm aghast at the tipping culture in the US but more importantly, I fail to understand why tips are calculated on a percentage basis. It is no more effort to bring me out a $60 steak than it is to bring me a $5 coffee. So why should I tip $12 for the steak and $1 for the coffee? To Apple's point, they want a "fixed" tip charge. But then…
You aren't paying for their effort in hosting the app. It's a fixed cost and the App Store runs largely on the back of the iTunes Store platform. You are paying Apple for being a distribution channel i.e. channel to market. And it's incredibly common for those to be based on a percentage rather than fixed amount.
Apple does spends a significant amount of money (far more than your 30%) on giving developers the perfect market to sell to. Every customer has a credit card, is on the latest OS, is willing to spend, is unable to pirate etc.