Because that picture that he knowingly took with those people wasn't meant to be shared with others and was just supposed to be looked at in private by the person that took it.
Here we go. Wrong.
I can't believe this is so hard for people to understand, and why it's so astonishing when someone points out what things like social media can do.
I literally just recently did a company-wide 'training' on the etiquette and problems with social media.
I'm talking about people that I was FB friends with, 'checking me in' if we were out to eat at a restaurant/bar, concert, whatever, when I haven't even been around them or talked to them the whole night, and most certainly haven't gotten a picture taken with them. I didn't catch the 'selfie' fever and find that to be pathetic behavior as well, but I'm sure that's too far of a concept in this discussion as well.
It's not 'invading' privacy like being snoopy; it's breaching in the sense of disclosing personal information without my consent: announcing one's personal life to the public. There are people in the world that think that's a normal concern, right? I hope?
Aside from valuing privacy, I happen to work in a field where being tagged/'checked in' at a bar can easily be seen as inappropriate, even if I was there early in the night. FWIW I've actually had to make requests for people to not photograph me or check me in since they couldn't figure out that doing that may affect other areas of life. I'm fascinated with people not being able to just be present somewhere without having to seek approval for it. That shows a complete lack of awareness and very poor social skills.