I think it is more about the scale of the response and the disruption of daily life that it caused. I remember all sorts of statements from officials after 9/11 saying things like how school, work, and general life wouldn't stop in the face of just this sort of threat. How people sheltering in their homes and not doing their usual things was in effect "letting the bad guys win." People at work were told they couldn't go home. People at home were told not to go to work. Door to door searches going on, the stopping and searching of any vehicles attempting to cross the perimeter established inside Watertown. A perimeter that by and large was apparently being maintained by the national guard so that police assets could be used for searching. It just seems like a potentially excessive response.
I understand the need for emergency workers and law enforcement to not be hindered in their tasks in responding to this sort of thing. But in the end, Boston was shut down for a day, major events and individual lives were postponed and delayed. The economic costs of this shut down are estimated to be close to 300 million dollars. That is counting the mobilization of national guard assets. The restriction of civilian traffic both on the ground and in the air. The closure of air space around Boston and the shipping/travel disruptions stemming from it. Not to mention the impact on local businesses that can and do conduct local, national, and international business.
All this to catch a 19 year old.