> Because what Apple is doing is not immoral or unethical

Prohibiting anti-censorship apps and content-based censorship of apps at the behest of undemocratic governments is not immoral or unethical?

> Every company who runs a marketplace sets its terms and distorts competition to benefit them.

Not every company monopolizes the distribution of a third party product to an identifiable discrete set of customers.

> In this case the author et al are arguing that Apple should be regulated differently and forced to operate a "level playing field" marketplace.

That would be a valid way to remove the moral issue by taking the ability to impose censorship out of the corporation's hands.