Funny. When they launched, everyone was expecting it to be 50/50, and in fact Intel and Microsoft had announced or launched stores with that as the split but changed to 70/30 as soon as the App store came out. Some rivals even complained that Apple was using it's financial muscle to undercut them at below-market rates as a loss-leader, yet even then at the same time others were making the same complaint you are.

Bear in mind the App Store was losing money or barely breaking even for most of it's existence. It's only in the last few years it's begun to make significant profits. They've sunk billions into it. That hardly seems like the business plan of a gouger.

As for being a monopoly. That's like complaining that the shop down the road is a monopoly provider of products in their own store. There is no monoploy, you can always buy an Android, Symbian, Tizen, or even Windows Mobile phone.