Well that's the thing with destroying evidence. If you destroyed it, it becomes harder (or impossible) to prove you did the crime. That's why it's not uncommon for people to be only charged with stuff like "obstruction of justice" rather than the actual crime they allegedly did.
And what if he was erasing a steamy affair with a border patrol agent? That's not illegal, but releasing knowledge of it could be damaging to all parties for no reason.
if the only evidence of a crime is on your phone, what kind of crime is it?
we should always be asking: is this the only way you can prove the accusation? just because it would make LEO life easier - that's not justification for violating the constitution.
an consider what this case teaches us: clean up your devices before you cross a border. how does that even help the goal of law enforcement?
But you can't assume someone committed a crime.