It's not true that Islamic terrorists only targets are politically motivated. Take the (failed) times square bomber. His intent was just to inflict massive causalities. Also note this incident in Pakistan a few days ago:
http://www.pentagonpost.com/peshawar-attack-bus-blast-kills-8-injures-7-in-pakistan/8346052
The bomb was on a civilian bus that was passing through a crowded market. Nothing to do with targeting government forces - the ideas is to spread panic and fear by attacking soft targets.
http://en.wikipedia.org/wiki/2010_Times_Square_car_bombing_attempt
"Shahzad reportedly had four other high-profile targets in the New York area he was planning to attack if his first attack had been successful.[89] On his list were Rockefeller Center, Grand Central Terminal, the World Financial Center (just across from Ground Zero) and the Connecticut-based company that manufactures helicopters for the U.S. military, Sikorsky.[89]"
I've never heard of Islamic terrorists targeting a city OTHER than NYC (okay, DC) in the USA. They've never attacked say, Philadelphia, Boston (before this), Miami, LA, Chicago, etc. NYC is the global finance capital of the world.
Islamic terrorists do blow up buses all the time in Israel. And they did attack the London subway. Still...this just doesn't feel like it's Islamic. Boston is just not that high profile a city to get the attention of Islamic terrorists. They tend to attack the gleaming beacons of the West, not sporting events like the Boston marathon.
On the other hand, we have a history of right-wingers attacking sporting events. Eric Rudolph attacked the Atlanta Olympics.
Also, Islamic attacks tend to claim responsibility soon after. Meanwhile, right-wing attacks they don't announce anything. THey sneak away. McVeigh and Rudolph behaved in this way.