Earlier quoted context omitted.
Make sure you include a few vanilla porn pics and only slightly embarrassing drunken party photos. If they don't find some evidence of vice, they will suspect it was just staged data, and they might keep digging.
When border patrol agents are looking for narcotics, do you honestly think they pass over the guy carrying rolling papers in favor of the one carrying nothing remotely suspicious due to lack of evidence of vice looking "staged" in the latter case? What's different about a porn- and photo-free hard drive full of boring business reports and uninteresting browser histories?
Among other goals, they are assembling profiles on dissenters, to be used against them later. If you give them something that appears legal but still potentially embarrassing, that's disinformation that might save you from a stronger attack later.
This differs from a narcotics search in that having data on electronic devices is not a crime. They could not perform such a search anywhere but at a border crossing.