A similar thing happened at my school a few years ago. A girl died at a swimming event and tons of people who didn't know her, or even any of her friends were crying and talking about it as if they did.
I guess maybe it makes people think that they aren't as immune from death as people seem to think they are when they are young. A lot of attention seekers jumped in on it too though.
If it's bothering you, just stop using Facebook or just try and shrug it off when you see it.
Personally though, I feel the same as you do. I don't get it when people are emotional about things that aren't affecting them. I don't see how it is of any benefit to get upset about something you have no control over or doesn't affect you. Instead of getting upset, I'd rather see people try and help those who actually are affected by whatever happened. That's much more useful to everyone.