Free Apps with ads in them

As an iPhone transplant I have always wondered why there are such differences between the apps I would get from the iTunes store versus the Android market. A lot of the apps from iTunes are more polished and then there are the ads. I know people are in the app business to make money, but is there a known reason why iTunes apps are available for a price with no ads, when the same app is free for Android with ads. The one that annoys the crap out of me the most is words with friends. The iTunes version was like $2, no ads and works like a charm; the Android version is free, but you have to see an ad after every single turn and will randomly drop games I am playing.

Does anyone know why this is? I would gladly pay for apps that I don't have to have ads in, but most of them aren't available for Android.