Contraband can be both physical and digital. If the government can conduct searches for physical contraband then searches for digital contraband in certain circumstances like border crossings seems reasonable.
This opens a door to unlimited search power at US borders against US citizens. There is already a 100-mile inland range where your can be searched without a warrant around our borders -- we are only going to make this worse by extending that range into all of our private digital data.
I posted a house/key analogy just now that may help clear up the distinction between physical and digital data search. Just to be clear, I'm actually ok with border guards physically inspecting my electronics. I am not ok with them compelling me to login to them, decrypt them, or otherwise hand them over my social media / banking logins / etc.
Keep in mind, we're talking about US citizens re-entering their own country here. I think this is a key point.
But there is no reason to search the devices of U.S. citizens and lawful permanent residents, who are already authorized to live and work in the U.S. These people would have no reason to break immigration law. And if the government is searching digital content for evidence of other unlawful activity, they should have to get a warrant.
Keep in mind that any digital content you can bring through a border port-of-entry, you can also just send it via the Internet.