They're not seizing anything. They're just taking a copy so they can search while you go on your way.
Such searches at a border for physical items make sense only because it's the physical entry/exit point, so you can make the argument that you can keep things in or out, or keep an eye on what is coming in or out. That simply isn't true when it comes to data. There is no physical entry/exit point for data, so "special" rules for border crossings for data serve no logical purpose.
