People may not spend to buy apps but they sure do spend once they are inside. The growth of such a new thing with so many developers building apps charging would have slowed the market to a snails pace. Also the amount advertising generated is pretty huge.
"The report, which has been compiled by the mobile team at technology analyst firm Screen Digest, predicts that in-app purchases will rise to account for 64 percent of the total app market revenue by 2015, up from 39 percent in 2011
Revenue from in-app purchases will increase to $5.6 billion in 2015, up from $970 million in 2011. The report estimates that 96 per cent of all smartphone apps were downloaded for free in 2011."
Source.