"You are free to copy, distribute, transmit and adapt our data, as long as you credit OpenStreetMap and its contributors. If you alter or build upon our data, you may distribute the result only under the same licence. The full legal code explains your rights and responsibilities."
If so, that's very nice but I am not sure that's what is actually happening: the third-party source whom they are licensing data from is unlikely to be happy with a scenario where their data ends up in the public domain as this considerably reduces the commercial value of their map data.
But Apple simply cannot force someone else's data into the public domain, if they were mixing the data without permission, they would be violating the licenses and legal action could force them to fix that (but it wouldn't directly change the licensing of any of the data)