What has David Miranda done which violates that law? Miranda was not charged with anything, he did not violate a law. He was brought in for questioning and seized of his property. This is allowed since the law is aweful and can be interpreted this way. If it was unlawful the prosecution could prosecute the police that did this. They likely won't since the law is fuzzy.
Again, I'm not supporting any of this. I'm just trying to shed some light into this since most comments her on HN are simply complaining about UK/US without digging deeper and asking themselves why this is possible and why nobody has been charged with anything yet.
Law are not black and white