Until we can find a better way to enforce civil liberties, the striking of illegally obtained evidence in the prosecution of terrible criminals is necessary. That they get to walk free is the point first as a penalty to the state (that now a monster remains at large) and second as a penalty to the public for allowing the state to let its agents abuse their power.
If neonazis and terrorists aren't protected by our Bill of Rights, then you aren't either. And it informs how the massive extrajudicial surveillance state got formed in the first place, as the US state believes national security (in all its ambiguity) is valued more than American lives.