Now I stick to puzzle games that provide a large amount of enjoyment relative to the cost. The Room series is phenomenal.
They open up pandora's box and allow game devs to focus on superficialities to increase revenue (such as a bonus level, ways to skip level, etc..). Were the playing field completely even -- no in-app purchases -- gameplay would necessarily have to win out and people might spend more time reading mobile game reviews.
When you can be angry bird and get on the top grossing list for a long, long time because you got in-app purchases, I believe it skews the data and as a result, many are poorly informed as to which game truly is the best.
But you've defeated your own point by using the word "allow", which while it it accurate, illustrates why it's not inherent. It's not inherent because they aren't forced to. Is it likely? Sure. Are the incentives all screwed up? Sure. But it's possible to use in-app purchases in responsible ways (such as purchasing future "episodes" of more story driven content, extra level packs, etc as long as the core game provided "enough"[1] content).
1: Which is entirely subjective, and I submit that some people that would have been completely happy with content delivered as is without the ability to download more for cost interpret the situation differently when they see extra content available.