I found the reaction of a lot of the witnesses curious, but it was curious precisely because it was not the reaction one would expect, regardless of whether they were British or American. To suggest this is a cultural thing is wrong imo. The British public's actions in the aftermath of the 7/7 bombings were pretty much the same as Americans after the Boston bombing, with people rushing to help the injured and others running away.
There must have been something about this incident which caused people to react in this unusual way. The craziness of the situation, the fact they didn't try to run away and were talking to witnesses, perhaps the public were trying to keep the situation calm and not provoke the attackers into any further violence, I dunno.