It's more about what they don't do to help developers make money. A web search will find research papers on software ecosystems, which discuss best practices for mutually-reinforcing, virtuous circle, feedback loops between platforms and developers.
it's not that they don't make money, but they don't allow developers to operate good businesses. being able to give trials to users, or to handle advertising in certain ways, or to have more flexibility with payments, etc - all of these things would be beneficial for businesses to be able to control directly, but they can't, as they're in the walled garden.
Many are making money right now, but I would suggest they don't have terribly good businesses, in that they don't own the relationship with the end user. One change of Apple's policies can put you out of business.
Apple could attract more developers (and thus more iPad users and more corporate revenue -- reversing declining iPad growth) if developer success was more closely aligned with Apple success.
Stardock's 2014 report touches on related topics, http://www.stardock.com/press/CustomerReports/Stardock2014.p...