Does that "build-upon" clause also apply for the case where a provider is mixing OSM data with other third-party licensed map data ? For example, it is mentioned/known that Apple makes some use of OSM data[1] to improve their maps in some places. Are they then legally obliged to release the full data behind their maps for those regions?
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.