The other thing he is missing is that they did not declare martial law. This was police work by local cops. A governor or President is required to declare martial law, and then, the military is in charge with the rules of war applying.
While I appreciate everybody's concern about serious constitutional questions with the way that the Watertown police handled the alleged bomber, I'm also concerned that delays necessary to comply with procedures may have resulted in even more deaths. After all, if I understand the following article correctly, I'm not sure of the condition of one Watertown law-enforcement officer who was seriously injured in confrontation with the alleged bomber.
Lone officer confronted marathon bombing suspects in firefight, chief says
Also, note the last part of Clause 3 of Section 10 of Article I of the Constitution.
Clause 3: No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay (emphasis added).
Although the clause emphasized above is not directely applicable to the Watertown situation, it reflects that the Founders understood that authorities may sometimes be forced to act because of time restraints.
I was getting ready to post but had an interesting insight. Note that the Founding States had decided that government prohibitions / limitations of power in the Bill of Rights did not apply to the states, only to feds. (14A changed that.) So 4A's privacy protection without warrant originally did not apply to states.