Originally Posted by crockett

It's because they probably are actual under cover officers, most likely part of the Boston Swat team dressed to blend in somewhat with the crowd.

Every major city in the US uses officers in these kind of situations that are in "street clothes" at major events. They are usually around to keep people from doing stupid things like planting bombs in crowds and even make drugs arrests. Things of that nature.

However soon as the conspiracy nut jobs see them, it's obviously part of the master plan..