For millions of people, it's their main/sole computing and internet access device so should be a neutral platform - with clear evidence as cited that Apple has not maintained its neutrality. As a neutral platform, customers should have the freedom to use their devices without undue interference or restrictions from Apple.
These are similar arguments made in the Microsoft vs. Netscape case. The lone example of being unable to install non-App Store apps is enough to justify the DOJ's case. Question is what would the verdict be? Similar to the EU's DMA rules would be a likely starting point.