People who say "Just an apple" obviously have no connection with, or comprehension of, primary production.
NZ does not have fruitfly endemically. Every now and then, a few are found: they may have come in by an improperly treated commercial shipment, or it might have been someone who thought "It's just an apple." When these infestations are discovered, it is treated as an A-grade problem. In the 1980s, fruitfly were discovered in the (rich) eastern suburbs of Auckland. Somehow, it was decided that the cost effective way of dealing with this was aerial spraying by a converted DC 6. I have my doubts, but it was certainly a stirring sight to see something with round engines laying real chemtrails at low level. More recently, some flies were found near me. A zone was set up out of which no fruit or vegetable matter could be taken. It lasted months, though after a couple of weeks the big supermarkets were exempted. Doubtless a few cackheads ignored it, but mostly people accepted it as inconvenient but important. That's because even in NZ's biggest city, we know how important it is to keep out pests, plague, and parasites. As for the manners and practices of US border agencies, I've had mixed experiences--though often, they're very friendly. But biosecurity is not trivial, and one apple can make a difference--not to mention the problems of galloping sorites.