The importation of immoral articles is illegal. Where pornography is concerned, this has been defined by the courts as bestiality and child porn. CBP seizes child porn quite frequently, and that is almost certainly what he was looking for. While some are good at hiding it, many don't bother and bring it across on hard drives, CD's, etc.

The "business records" part, however, does not make any sense.

And yes, he had every right to search your stuff without you watching.

Edit: bestiality is in a weird "you can possess it, depending on state law, but importation is still illegal under federal law" category, along with switchblades.