If I want to purchase a music artists album, I can walk into multiple brick and mortor stores or from any of the multiple online music stores.
The artist/record label is perfectly free to make their new release (for example) an iTunes or Spotify exclusive, or only sell it through their own online store. Where that has been tried by a major act, it has got a pretty bad reception from customers, but it wasn't illegal.
Alternatively - good luck watching the new
Stranger Things without subscribing (directly or indirectly) to Netflix.
Neither of those is quite the same as the App Store situation (where Apple is saying that to publish on media X you have to sell through our store) - although there have been plenty of record/tape/MP3 players with proprietary media formats (go watch TechMoan on YouTube).
If I want to purchase a console game, again I can walk into multiple brick and mortor stores or from any of the multiple online gaming/retail stores.
That's a really bad example when many console platforms require developers to licensed and approved by the manufacturer. Even then, the only reason you can buy software in shops is because the publishers have allowed it (they don't have to but it's usually bad for business not too). Also, if you've bought software in a store recently it's often more like buying a voucher that lets you download it from the publisher's website - only one step removed from buying an iTunes voucher.
But if I want to purchase an app for my iphone I have only one option and that is Apple's app store.
...and you knew that when you bought an iPhone instead of an Android phone.
So why is it that manufacturers, companies and businesses are constally being told they must allow others access to their services, products or parts but yet when it comes to Apple it is allowed to restrict how the app store opperates (no 3rd party app hosters allowed, no sideloading allowed, no other online outlet allowed to host or sell Apple apps)
...because it's not illegal
unless the company is deemed to be abusing a dominant market position, and
that is a subjective question that can only be decided by paying lots of money to lawyers, lobbying government etc.
The other thing is that Apple are being accused of having a dominant position in a market (mobile phone Apps) which they played a major role in creating from scratch (either with the iPhone or earlier with iPod/iTunes) - whereas one of the key no-nos for monopoly abuse is leveraging a dominant position in one market to muscle in to another.