Given the extremely high profile nature of this entire scenario, I don't see how anyone on the case could expect the probability of Greenwald using his partner as a data mule to rank as "very likely." I'm not saying it's impossible. Just that anyone who's been paying attention for the last decade or so knows better than to travel with hardware across unfriendly borders like the US or the UK. They're just going to take it anyway and then you're out thousands of dollars of gear. Between Snowden and Greenwald and Poitras and The Guardian and Wikileaks, I'd sincerely hope that they'd be capable of utilizing dead drops online.
In my opinion, this "retrieving stolen property" argument is merely the parallel construction of the actual motivation: to intimidate journalism. They know perfectly well that the probability of Miranda carrying anything actionable on him is minimal. But that's not the point. That's the pretext. The point was to pressure cook him for nine hours in order to glean from him any useful tidbit of intel, with the understanding that even if nothing comes of it, they've sent a clear message to The Guardian that they're taking the fight to them personally.