Thus far Apple Maps doesn't have ads. There are rumors they may ad them (pun intended), but I don't think their motivations for steering people towards Apple Maps are primarily monetary.
1. Using Apple Maps makes the switching cost to other devices (that don't have Apple Maps) higher.
2. Having more users makes any future monetization more valuable. I understand that there doesn't yet appear to be any direct monetization but I very much expect to see it at some point.
3. Removing traffic from competitors hurts them making their product relatively better the the competitors.
Same thing with Apple operating iMessage for free without ads... they don't care about monetizing iMessage but it's also not about altruism.