Now it will be "before upload". In 1-2 years it's "scan all local photos" in the name of "make the World a better place". It's such a small technical step for Apple to change this scanning behaviour in the future and scan even offline photos. All the necessary software is on all Apple i-devices already by then.

Everybody is a potential criminal with photos on your phone unless you prove otherwise by scanning. This is the future we are heading to. To do the scanning on device is actually the weakest point of their implementation IMHO.