When you have Apple software the only way to download and purchase apps is through their app store.. Do you think that it is a monopoly?
It's not a monopoly- you can get the apps at other places too, like for Android. It's part of the Apple SYSTEM of devices, software, hardware, media and the way they all work together so smoothly.
I think it's more similar to complaining that you can only buy Sams Choice cola at Walmart. That's not a monopoly, it's their version of offering you a coke alternative. You can buy coke anywhere (like a CD or DVD) or if you want the store brand you can buy Sams Choice or Thunder (Walmart Brand or Kmart brand, as Apple or Android apps).
You can buy DVDs, CDs, or watch the movies in theaters and the shows on TV- it's not a monopoly. It's Apples system to make and sell products, and the way they keep it (what seems like) a closed system, is really just how they keep it working so smoothly, and can charge a premium for the smoothness of the operation of it all.