What most people are missing is that they don't need a warrant because you're outside the U.S. wanting in. Supreme Court cases have established that U.S. Constitutional protections apply only to people (both citizens and non-citizens, including illegal immigrants) on U.S. soil.
If he surrenders the laptop, what right does immigration have to prevent a US citizen from entering the country? Surely, he has a presumtive right to entry when he is at the border.
That's why Bush built a prison in Guantanamo Bay - that's Cuban soil, not U.S., so prisoners there wouldn't be protected by that pesky Constitution.
Could I murder someone in an airport before going through immigration and not be prosecuted? After all, if it is not US soil, then US laws don't apply. There is a world of difference between Gitmo and an airport.