You don't seem to comprehend it's
not about a monopoly on "smart phones" but about a monopoly on software distribution for a given computer system (iOS). In other words, the "monopoly" is on SOFTWARE, not HARDWARE. Imagine if Microsoft told you that you could only buy Windows software from them and charged 1/3 your profits for every piece of software made for Windows. That's essentially what Apple does for the iOS operating system. People used to say it was just a phone, but iOS extends to everything from tablet computers (even 'pro' versions) to tv set-top boxes (AppleTV) these days. It has arguably become a general computer operating system at this point (how much more so if it ever merges with 'macOS').
It actually breaks the TYING clause of the Clayton Act. Artificial 'tying' (meaning you must buy one to get the other when in fact they are two separate items/markets) is PROHIBITED when it "substantially" lessens competition (
https://en.wikipedia.org/wiki/Clayton_Antitrust_Act#Contents). In this case, every time Apple denies an app because it competes with Apple's own app, they are violating the tying clause of the Anti-Trust Act while their App Store denies ALL (reasonable) competition for DISTRIBUTION of software for the iOS market (having to 'hack' your phone to install 3rd party software is not 'reasonable' and severely limits the software available through such channels).
The ONLY legal question here is whether Apple's iOS products represent a "substantial" market (i.e. is competition being substantially reduced as a result?). There is NO QUESTION they are violating the literal interpretation of the Clayton Act's tying clause by forcing you to buy all your 3rd party software through their market place and they are violating developer's rights by denying them distribution because it competes with their own software.
A similar argument can be made in reverse with the Mac hardware market. Here, Apple is forcing you to buy their PC hardware in order to run OS X (macOS). They've artificially TIED the hardware to the software, meaning if you want to use OS X and software designed for OS X, you MUST buy an Apple brand computer even though it's internally no different than any Windows/Linux PC hardware out there. In the past, one could argue the Mac "market" was not substantial compared to the Windows PC (given rulings against Microsoft for having a functional "monopoly" despite the availability of OS X and Linux among others for such hardware). Apple was a company on the brink of bankruptcy in the late 1990s and so its "market power" was not substantial. However, I would argue things are a bit different in 2017 than in 1998! Macs represent a substantial hardware seller in the market place (even for those that might buy it to use mostly or all Windows on something like a Macbook Pro). They are no longer insignificant in hardware sales and the company is one of the largest tech companies in existence. The idea that the Mac is no longer a substantial market is absurd. Thus, I would argue that Apple is in violation of the Clayton Act tying clause for HARDWARE for OS X (macOS). They are forcing you to buy their PC hardware instead of something from a competitor like HP, Lenovo, etc. in order to use OS X and its software market.
The SOLUTION for the Justice Department in BOTH cases is simple. BREAK THE TIES. Force Apple to allow 3rd party software distribution for iOS and allow OS X to be purchased separately and installed on non-Apple hardware. The CONSUMER should decide what product merits purchase for hardware and software separately as they are two different markets as evidenced by the FACT that OS X CAN EASILY be "made" to run on generic PC hardware (i.e. Hackintosh). There is no technical reason it won't run other than Apple purposely blocking it and not selling it separately.