The gross invasion of privacy is not taking pictures of people in public spaces. It's cross-referencing those pictures against images people have uploaded to various online services, many of them before this kind of use in facial recognition databases had even been conceived by anyone except science fiction writers.
When lone individuals go trawling social media sites for pictures of some random stranger they met on the street, this is commonly understood as creepy at best and (especially to the extent they are violating the terms of the platforms) an invasion of privacy at worst.
