That may not qualify as a "relevant market" for determining monopoly status. (The Conclusions of Law from the MS v DOJ case makes interesting reading along these lines).

http://www.justice.gov/atr/cases/f218600/218633.htm

Whether this zone of commercial activity actually qualifies as a market...depends on whether it includes all products "reasonably interchangeable by consumers for the same purposes." ..."Because the ability of consumers to turn to other suppliers restrains a firm from raising prices above the competitive level, the definition of the 'relevant market' rests on a determination of available substitutes."

So if a plaintiff were to try to argue that Apple is the sole supplier of App Stores for iPhones, it would become germane that nothing is charged for access to this service, and that if access became expensive consumers could flee to other smart phones.