I'm not sure where I stand on it, but I do see a qualitative difference if the government were routinely using cameras in public areas with facial-recognition software to track people's movements around town, versus just using the cameras as security cameras. Given good enough computer vision, and ubiquitous enough cameras, the end effect can be very similar in tracking effect to the government being able to attach a GPS device to you.

It's true that it's all pieced together out of snapshots that would individually be unobjectionable (someone taking a photo of me in a public place), but the end result seems rather different. If a private party attempted to do it, we'd normally consider it a form of harassment, and it'd be illegal in a lot of jurisdictions. You can photograph someone in a public area, but you can't hire an army of photographers to photograph them all around town as they go about their daily routine and plot their movements on a map, because that'd be weird/objectionable.