Earlier quoted context omitted.
Because a market is suddenly opened for business, alternative payment processors can now start offering systems that take care of this, and with different terms from Apple's. You wouldn't have to handle all of it yourself, you could simply sign on with one that provides you with service you like. This is the free market competition that tech is supposedly in favor of.
And these services would all take a 15-30% cut. It wouldn't be profitable otherwise.
The 15% pay cut is an industry convention, but is that actually justified by any financial reasoning? With actual competition in this space, business models can be discovered that could potentially lower costs. Not to mention, given the VC effect, there will likely be newcomers that try to undercut Apple/Google and each other with lower prices by burning investor capital, for a time.