Tsarnaev and his brother planted bombs. It is not unreasonable for the FBI to be concerned that they may have left other bombs in place that could detonate later. Or, that there were other people involved that we don't know about. Invoking the public safety exception to the Miranda rights seems prudent to me.
Be VERY, VERY careful before you throw out that distinction. One day you may find yourself arrested for something you did or didn't do, only to find public opinion very much against you based on little more than hearsay.