Never underestimate the power of the mob mentality. There are so many people out there who can't think for themselves - who can't stand back in that situation and make rational decisions. The lure of going along with what the mob is doing is strong.
But, to engage in such hooliganism and not realize that everyone has some sort of camera, and that your photo will be taken by at least one person, more than likely posted on at least one public website? How ignorant can they be? That water polo kid… how could he not know that he would be caught?