Well, being listed on a price comparison website is basically advertising, and can funnel a lot of traffic to Amazon if they are often the cheaper or better choice. This service just used referrals to make a bit of money instead, probably because there's a lot more price comparison website competition in the US and Amazon doesn't want to fund them all to get listed.
They get away with it too, because a price comparison website is not feasible without having amazon as one of the shops.
They get away with it too, because a price comparison website is not feasible without having amazon as one of the shops.