“In the United States, Apple iOS devices — those running on Apple’s proprietary mobile operating system account for roughly 60 percent of mobile-device usage. Apple’s is a closed ecosystem; Apple does not license iOS to third-party mobile device manufacturers.
Another roughly 40 percent of mobile-device usage comes from devices that use Android, an open-source mobile operating system controlled by Google. Unlike iOS, Android is licensable, which means third-party mobile device manufacturers can use it as the operating system for their devices. All other mobile operating systems, combined account for less than one percent of mobile device usage in the United States.”
(Of course, whether that’s correct is a different question from whether that’s what the DoJ claims, but it doesn’t seem farfetched to me that it is correct, too)