But he didn't buy anything! This is like walking into a book store, looking around at books, reading a bit, and leaving the store without making a purchase. Clearly a federal crime.
One person taking a book means others can not see the same book. Acceptable DoS because the owner thinks you may purchase it.
One person taking several books means others can not see the same books. Questionable DoS because the owner thinks you're screwing around.
Many people doing taking several books out in the store leaves the owner to suspect a DDoS attack because there's not usually that many people and none seem to want to buy. It's especially malicious if there's intent.
Ugly thing about DDoS is that since there's unexceptionally large amount of people and no books to check out, loyal and new customers will be put off and may not return causing future loss.
They're going after these people to make an example of them, not because they did anything actually harmful.
Probably a crime, but a pretty minor one.