This is actually kinda old news. This topic came up in 2008 by CBP with laptops and cell phones being searched by border. Also, the 5-day rule is not necessarily applicable since they can cite "National Security" and keep the phone longer if they want. There is no way they will only hold it for 5 days if they state National Security concerns.
We posted some material on our business facebook page about 4 months ago that indicates how you can protect your personal data during these searches of phones and laptops. They have always been searching phones during secondary searches for entry into USA. They even literally searched mine in 2008 at the Pacific Highway border. Good info but it is something that got really big right after 911.