When you put into context what happened, maybe you would understand.
The Boston Marathon isn't a school, or a movie theater, or a financial building.
These are people from all over the world, sponsored by and running for very good causes; They run for cancer research, world hunger, peace, equality, human rights, and it takes them years to train to such a level, and a huge amount of committment. The run is so taxing on the human body that people literally shit themselves on occasion.
And everyone in the crowd is there to lend their energy to the runners, to congratulate them, and to show their appreciation.
People in the crowds are internationals. They fly here from other countries for what is called one of the toughest marathon courses in the world, and we are such a melting pot already, that likely every single country has runners and support in the crowd.
So yes, it is in very poor taste. Not from an ethnocentric point of view, but from a worldview.
But if there's one thing, we Bostonians are a bunch of tough mofo's, so we'll get over this incident and the "jokes". Why? We have a saying here, and it goes something like
"because fuck you".