Because they had committed the cardinal sin- they made those in power look bad. Across the US, most of us hear the Attorney General's pitiful "it was a sinister device. It had batteries and wires!" excuse, we realize that the perpetrators of this crime are doomed, doomed, doomed. Not because they caused a public nuisance. No, that responsibility rests squarely on the shoulders of the Boston PD. No, they induced the government to overreaction through their irresponsible use of cheap electronics to make simple displays. And thatwas their crime- they encouraged the government to make itself look bad.
I think they can expect, and deserve, a death penalty for their arrogance.