It used to kind of make sense when they were just searching physical property (baggage). There is a legitimate argument for wanting to stop dangerous or immoral things from entering any given country (weapons, poached ivory, foreign animals that may disrupt the ecosystem, unsafe food/drugs, etc).

Then there are financial reasons for stopping some items that have tariffs or duty on them. Then there are legal reasons for stopping other things that violate the law (items that violate local copyright/patents, etc).

I think most people would agree with those items above, so they have a legal right to search you and your belongings. Those same laws have been applied to computers and phones without any new thought. So, if they have to ship it off to a lab to search it because you won't share the password then they have the legal authority to do so.

I don't agree with all of it, but they have the legal authority to do it.