If they had ignored it and it had been a bomb, what would we be saying today?
Well of course we'd be condemning the cops for not doing enough. But that's the point: The police seem to think they have only two options, ignore or overreact.
The proper response in this case would be to ask the girl what the device is and after a satisfactory explanation, let her go with it. To arrest her for non-criminal action s and charge her with a crime that requires intent when there clearly was none is ludicrous - and unfortunately, is a recurring theme with the Boston police.