I guess they could. But since they have to do everything themselves anyway, why bother? I'm not sure about the legal aspect, but even so, I'm pretty sure Apple can use OSM's data in one way or another would they need to (in fact, article states that they did use it in the earlier versions of the map). And since they are making the map on their own, nothing forces them to open-source the DB right now: should they decide that it will be beneficial they can do it any time in the future. And should they decide it isn't: they just don't.

Yes they could. The OSM licence requires a share-alike, so OSM would be able to use anything Apple add, but all that's required is a database dump. It would be up to the OSM community to "merge" it back in if they wanted.