They did it when they knew what area to lock down, didn't they? As in, when they thought they knew where the suspect was. Turned out the suspect was just outside the initial Watertown lockdown area (so it's a good thing they quickly expanded the lockdown, and it shows they got the area roughly right).

I agree with you in general. Seemed like a good call, not a sign of a police state. These were extremely temporary and timely measures, and normality resumed quickly.