No, devs don't buy customers from apple. Customers buy apps, and Apple takes a commission. No money flows from Developer to Apple (apart from to 100$ annual fee if you want to be anal about it).
No they buy access to customers. You can't sell to Apple's customer base unless you give them 30% of gross. The world where Apple charges that 30% commission to the devs after the sale or collects it from the customer during the sale is irrelevant. We fork over a lot of money to Apple for the privilege of selling to their customers.
Well, you can't sell peaches to Walmart customers without Walmart taking a cut either. But we don't say the peach growers are buying customers from Walmart, do we?
I think that nowadays most profitable software is cross-platform and is also available on other platforms, thus their developers see access to the iOS market as added value rather than their primary customer acquisition channel.
