Because that's what public space is? We've always held that principle, and I don't think 'reach' should affect that. If someone takes this to the extreme (i.e. follows you around in public, taking thousands of pictures and uploading them in real time) they can be charged with stalking, harassment, or a similar offence.

To turn it on its head, if you cannot take photographs of people in public without their permission, then we basically lose the ability to take any photos of public space.