In theory, yes, but in practice they can do whatever they want, and suing them after the fact is going to be expensive, and have a high probability of not working out for you.

When interacting with border officials (or any LEOs, for that matter), be polite, don't get hostile or aggressive, but also be firm and don't volunteer any information that you're not required to give.

Not unlocking your phone is probable cause. Non-criminals readily submit for inspection. (We can fix this by refusing inspection even when we are not criminals)