What is 'unreasonable' in the Fourth Amendment?

The following is not intended to be partisan in any way, and neither for or against any particular candidate, government official, etc. I don't share my political beliefs in these types of forums, so nothing of a political or partisan nature should be read into this post.

The Fourth Amendment to the US Constitution reads as follows:

"The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."

While a case can be made that even a physical search of laptops, luggage, etc. prior to boarding a flight violates the Fourth Amendment, there are few that would want to fly on an airplane whose passengers' luggage was not searched. To me, airport security checks are a 'reasonable' type of search and are consistent with both the text and spirit of the Fourth Amendment.

However, the US Federal courts have recently ruled that anyone entering the United States is subject to a search of the files on their laptop computers or other mobile devices. Is this unreasonable? I think so. Such a search, without probable cause, is clearly in opposition to the intent of those who wrote the Fourth Amendment. While the upside of such searches is that they can catch child pornographers, the downside is that your confidential records are now subject to inspection by TSA, customs or other officials without any sort of probable cause, warrants or any other protections that used to protect you.

The easy way to avoid this kind of search is not to carry confidential files with you. While this is inconvenient, it does protect your data from searches, as well as from data loss in case you lose your mobile device. You can use hosted services or other remote data stores to house your data while traveling, but this is not always as convenient as having the files stored locally.