Not to make light of a terrible situation, but I couldn't help laughing at a reader's comment on CNN's website earlier today where he/she made that exact point, but then noted that planting bombs at the finish line of a marathon would end up killing and injuring more Ethiopians and Kenyans than anyone else.
< not laughing .. seriesly >
The time could matter, I have no idea if it went off when the frontrunners began crossing the finish line or later. I also don’t know if any assumption about certain groups being historically better in a marathon still holds water if it ever did. There may be more purpose than is obvious, is all I was trying to point out.