I think it's kinda like Google working with Android as an open source project. Google makes money on Android primarily by using Android as a means of delivering the Google ecosystem. They don't care that it's open source because they don't make money by selling it.

Apple could do the same thing with maps. Support a robust open mapping initiative to make sure their primary business (selling iPhones and other devices) has a solid platform for emerging technologies like AR. Making sure they have a solid maps offering supports the Apple (aka iPhone) ecosystem.

Also similarly to Google, they can still push plenty of proprietary things on top of the open parts of the initiative.