There have been statements by Teresa May attempting to justify the use of the powers since possession and latterly publication of confidential documents could arguably aid terrorists, but Miranda's actions are clearly not within the conditions that apply.
So it is a case of the UK overstepping the mark legally in an attempt to buddy up with the US, again (the White House has confirmed being notified of the action, and thus did not prevent it).
In reality it may be invoking the Streisand effect and fueling opposition at least as much as it is suppressing free speech - after all, it is fairly clear that Greenwald et al take precautions (as they must have to) to ensure that documents are replicated and available in multiple jurisdictions. But there is perhaps still not a good understanding of those processes and the ways that digital data can move at a UK political level.