Semantics.
The first city settled in the America's was not st Augustine either.
We are talking symbolically here. Islamic radicals are obsessed with symbolism.
In their thinking, Boston is the birthplace of the USA as we know it.
Much as Mecca is the birthplace of islam( symbolically), the ral birthplace of Islam is medina, but there you go.
Rightwing domestic terrorists like symbolism, too. Other people have pointed out how many rightwing symbols surround this event in Boston.
For another thing, Boston is FULL of leftists and near the massacre in Newtown and generally in favor of gun control.
It is very possible this is a rightwing gun-nut attack, so don't assume prematurely that it's the usual Muslims. Remember Timothy McVeigh. People thought it was Muslims then, too, but it wasn't. If this country starts to split, and you all know it is RIGHT on the edge of splitting, we are deeply disunited, terrorism will be part of that as it always is in revolutions.