Out of the box those devices act as digital TV and radio receivers.
In fact I have been using them for just that for many years. Weird that someone can get in trouble for having a TV/radio receiver.
The person who was arrested was using the device to probe stuff that the Tunisian government doesn't want to be made known. If he had been using binoculars and a notebook at the airport, he would have been arrested too.
They were watching him, knew what he was doing there, and used whatever pretext was at hand to arrest him upon arrival (which suggests he had been there before).