Electronic Frontier Foundation:

Our lives are on our laptops – family photos, medical documents, banking information, details about what websites we visit, and so much more. Thanks to protections enshrined in the U.S. Constitution, the government generally can’t snoop through your laptop for no reason. But those privacy protections don’t safeguard travelers at the U.S. border, where the U.S. government can take an electronic device, search through all the files, and keep it for a while for further scrutiny – without any suspicion of wrongdoing whatsoever.