The answer is actually far more simple. They charge more because they can. It's a very common practice and there's nothing all that wrong with it.
The average price of an iphone is higher than the average price of an android phone (because of the low-end android models). Therefore, the average iphone user has more money to freely spend on entertainment (note again: AVERAGE, I'm not saying Android people are cheapskates).
You can find lots of market research and graphs that outline the difference in revenue on appstore and google play. Over here where I live this is very common practice. If I book a holiday for $500 including a flight, that same holiday would cost me $560 if I had visited that website with a Macbook. Their thinking is "You can afford a macbook, so you can pay more for this".
You may find it vile, but it's the way the world works