In-app payment policies are a major restriction. Apple and Google have unlimited access in offering digital goods and payment methods. Third-party apps are limited in scope and taxed 30%.

If two songs are sold on an iPhone for $1 each, one by Apple and another by [insert music sales app], the small app makes 0.70, while Apple makes 1.30. Lets say each has to pay 0.50 to the label for the sale. The small app profits 0.20 while Apple profits 0.80. That's a 4x advantage for essentially the same purchase.