We need a protocol for travelers who expect this to happen. Something like this:
Before traveling, consider that unscrupulous U.S. Border Patrol Agents and their TSA buddies will likely review the contents of your device. We can expect them to abuse this power. We can expect some of them to access bank accounts, private photographs, confidential communications. Therefore we can do the following in advance of passing through the actual border:
(1) Disable password protection altogether before you get to the airport or border crossing. Consider deleting your thumbprints from the phone's archives. They are going to have your password anyway so dumping this feature will prevent you from being forgetful about using a universal password.
(2) Delete all banking-related apps from your phone. These are dangerous anyway. Be sure to include investment accounts.
(3) Disable iCloud or other remote storage services. Again, these are dangerous anyway.
(4) Make sure you don't have any nudes of you or any loved ones in your picture archive. This is the first thing these people will look for. Doubly true if you are an attractive female.
(5) Clear text history, browsing history, and any purchase histories. This is just flat out none of the government's business.
(6) Email gets tricky, because many people (for example lawyers) have protected or legally privileged materials in their email. This is actually an interesting question that might take a federal case to resolve: does the Border Patrol's right to review emails trump the attorney client privilege? That's a really good issue to use to attack this abuse of power. For now, consider disabling the phone's ability to contact your email service remotely by removing your Gmail or Outlook account data before crossing. Especially true if you have brown skin or a weird last name.
The fact that any of this is even under discussion should be an excellent incentive for you to do THE ONE THING THAT CAN STOP ALL THIS, which is to get your happy ass down to your local polling station to vote these scumbags back out of power as soon as we have a chance. Did you vote last time? If not, this is on you. We can elect people who will end this. This is a big issue, one that people will run on.