Your logic is quite flawed. Nordstrom does not have a duopoly on retail clothing. I'm pretty sure if Nordstrom controlled 60% and Kohls had the other 40% marketshare, and they attempted to exert unfair control things would be different.
Bottom line is smartphones are now considered a basic utility (like phone service, power, etc. ) all those industries are regulated since consumers wont have choices, for example if i'm unhappy with my electric provider, I don't have other options. Apple made a business decision to lockdown the $1k phones we bought and prevented customers from being able to download and install apps unless we go through their app store. With this in mind, they are charging app developers a highway tax to be able to install on our phones. They can also remove apps based on whatever they want. This is too much control. If we had alterative app stores on our phones, we may be able to buy apps cheaper on another store, vs. apple's app store. But we have no choice.
With apple controlling over 55% US smartphone and Google with the other, they are a Duopoly and are highly subjected to regulation.
The question I always ask is, why am I able to install apps on my Mac or Windows OS without being forced into the MS app store. There is no reason this can't work on iPhone. Apple has made a business decision to make it this way, and they are abusing it. This will not last forever.