Look. We don’t shut down cities for ordinary homocides because those killers target individuals. A spree killer or bomber is targeting a whole population. Especially if they’re going after a high-profile (and high-security) event like one of the world’s biggest marathons. The intent is to kill as many people as possible. Even if the attack doesn’t kill that many people (as I mentioned on Twitter, the Molasses Flood in 1919 killed more than the marathon bombing did), that doesn’t change the intent.
If you let them go, there’s a high probability they’ll do it again. Especially in a terrorist (and let’s be clear — you bomb a marathon to incite terror, whether you have a political motive or not) attack. Because, again, the proximate goal of someone who bombs a marathon is just to kill as many people as they can, and that’s true regardless of whatever their ultimate goal was.
And even with all of that being the case, law enforcement didn’t issue the lockdown notice until after the suspects in the bombing initiated a firefight in a residential neighbourhood. Dangers don’t get more clear and present than that. It would have been irresponsible of the authorities, knowing the suspects were at large, armed, and willing to endanger or kill civilians to protect themselves, to tell the city to just go about its business as normal. If nothing else, it denied the suspects hostages or targets of opportunity.
So, if your first thought in this situation is “ZOMG MARTIAL LAW” or “ONOES THE POOR ECONOMIES”, would you kindly go fuck yourself?