This is the ultimate example of how a few bad apples can ruin the whole bunch. It is true that real child pornography is the objectification of a real flesh-and-blood minor, but in the case of owning manga and other comics that include child nudity, one simple question should be asked:
Who's the victim?
It seems like a victimless crime, or only a crime with a potential victim. In that respect, is it any different from attempt to smuggle weapons? Man, the line is so blurred here it's not even funny.
But customs officials really should not concern themselves with pornography at all. Lack of focus is the problem here, and their concern should be keeping weapons and drugs out of the country, plain and simple. Local governments can deal with pornography issues, as even if it is a flesh-and-blood kid, pictures and drawings pose no danger to society as a whole, other than what they suggest about the individual who possesses them.