The bomb was in a black backpack, and Dzhokar was wearing a white backpack. The DOJ just wants to throw the brother under the jail because they gunned down the guy who actually did the crime (Dzhokar's brother).
And it's wrong for the same reasons as it's wrong to detain muslims linked to terror plots on flimsy evidence. The proof is in the pudding; if they had anything, they'd charge them.
And they have been for years. Even though the braindead left doesn't realize they are already blown out of the water by comparative advantage, the general solution to the 'Cake cutting problem' makes it clear that statism is wholly unneeded to solve it.
In fact, market pricing mechanisms over time guarantee a situation much like the generalized solution laid out here; supposing there isn't third party interference such as provided by the state.