David Miranda's arrest was clearly a by product of his relationship with a journalist involved in the NSA/Edward Snowden leaks. Miranda was detained under an anti terrorism law, something that was clearly a stretch considering there has never been any such suspicion before. Miranda was clearly a pawn in a political game.
It is not right to detain someone because you think they may have done something wrong. He was detained for carrying "political documents" and accused of espionage. This is just an excuse for authorities to abuse their power. They need to have proof in order to be able to detain someone.