They are trying to get new people involved to hook them into the hobby and of course get them to buy more stuff from them. Remember you need an SD card, power, keyboard, mouse, oh look it's a RaspberryPi book, oh cool, they have a magazine and suddenly that $5 loss has turned into $80 of sales.

The person buying 10 at a time is just buying the boards, so no extra items other than maybe a PSU. If you really want 10 at $5 go every day at lunch, that's what I did.