The object perhaps was the average guy, the average American. That was the target in the failed Times Square attack too, average people in a very famous 'American' place. Times Square and the Boston Marathon are both things that would appear on the front cover of a 'Visit American' travel brochure.

In addition maybe the timing has something to do with relaxing as an event winds down. I read that is the best time to hit a patrol, when they are approaching the end and beginning to relax. This whole thing seems a cut above average in execution.

I am not trying to be contrary for its own sake, just playing devil's advocate.