Been happening for years.
I had my laptop taken on a flight into the US back in about 2010; they insisted it was started up, logged in, and then without a word walked off with it for about half an hour before returning it without a word and sending me on to be further processed. I presume they were duplicating the hard drive data or something. That was also the trip where - as a foreigner - I was "randomly selected" on each internal flight for further checking. I would have been more offended if I had not (a) had nothing of value on the machine anyway, and (b) feared they would abuse their powers by kicking me out of the country or locking me up on a pretext if I said anything. Needless to say, I've avoided the US as much as possible ever since and always flown via Asia.