But Apple's customer base is more valuable, which is why people still publish there despite the cost. Apple and Google aren't selling infrastructure, they're selling an audience.

That's fine, it makes sense to me that you would develop for Apple before Android because those customers are more affluent and willing to spend on software.

However I question whether the App Store is connecting you to those customers in any meaningful way when there is so much competition in there. Sure if you are lucky enough to get to the front page then you can do OK but most don't.

Apple also has a large and growing editorial staff writing articles and highlights for their App Store; it's virtually becoming a house magazine. So being in their store potentially offers exposure and marketing for free that's immensely valuable. (Whether it's 30% valuable is debatable, of course. Personally I think they should drop it from 15%/30% to 10%/20%, but "Apple: always more expensive than you think they should be" has worked for them for a long time.)

potentially is the key word here.

If you are basing your business on getting exposure in the App Store then you are building it on shaky foundations.

There's a reason I put it in italics, to be sure. :)

BlueSnap/Avangate/FastSpring develop a free OS that's used by 85% of the smartphones in the world?

The fee is paying towards the development of Android. Your comparison of "same services" isn't remotely valid.

Is it? I though Android was subsidised by advertising.

Payment processing is the smallest part of what the Google and Apple App stores provide, your comparison is ludicrous.

What other function do you think they are providing for the average developer?

Distribution is the biggest part.