Evidence of the four horsemen[1]. Child porn, terrorism, drugs, and the all-encompassing "organized crime" — which can include money laundering, which in turn can mean having enough money the government wants to steal it.

[1] https://en.wikipedia.org/wiki/Four_Horsemen_of_the_Infocalyp...

> but it's not obvious to me how searching electronic devices achieves any reasonable objective related to customs law.

https://en.wikipedia.org/wiki/Bernstein_v._United_States

> Yes, but electronic devices can't digitally contain physical substances that might be subject to customs laws

Child porn is prohibited to be imported in any form, including digital. Customs isn’t about just physical objects.

Of course not, but there is a degree of proportionality here. In theory a person's personal diary, letters, chequebook etc could contain evidence of crimes. However it's generally not been considered appropriate to read through them during a customs search.

The rights granted under the constitution were provided explicitly under the knowing assumption that they could be used to conceal actual crimes or criminal intent. That's the point of them needing protection from the authorities. The mere possibility of committing a crime isn't enough grounds to invade a person's privacy to that degree. You need actual reason to suspect it in an individual case to go that far.