Mobile OS's are more than just a "distribution channel" though, because there's a significant amount of lock-in that prevents customers from buying your software from another distributor even if they want to. I think a better metaphor is like this:
A billion people live in a company town ("Appleville"). When they first decided to move there, they paid a lot of money to buy a house there, and in return they get access to all the benefits the town offers - nicely-built houses, well-maintained streets, and access to lots of public services. Because living in the town is so nice, houses there cost a lot more than they do in other areas.
While you live in the town, you've signed a contract that says you're only allowed to buy things from one store owned by the town's developer. Most popular items are available at the store, so it's never too much of an inconvience, but sometimes things are more expensive than they are in other towns, or they aren't available at all.
Every time you drive into the town, you get stopped, and a bunch of guards search your car. If they find anything in your car that you didn't buy from them, they take it, and you can't have it back.
If you ever do want to leave the town, not only will you have to buy a house somewhere else (and your existing house will be worth a lot less than what you paid for it), but you're not allowed to take any of the stuff in your house with you - it all just disappears forever.
Now imagine you're a new company trying to sell a product, and the store owner won't let you sell your product in their town for some reason (you aren't paying them enough, it violates one of their rules, etc.) Even if the people living inside the town want your product, they can't buy it unless they completely uproot their life and move somewhere else, which no one is willing to do. And so the town owner is able to kill your product, and prevent it from ever being a threat to them. (Meanwhile, the town owner copies your product and starts selling it in the store themselves).