Well they are placing an increased load onto the side and get nice resources for free, not even doing the cheapest form of flattery: attribution. That's happening a million times on the internet each day, where pictures and other copyrighted materials are redistributed without consent or attribution, and it's ridiculous that the currently most valuable company on earth feels the need to be a cheapskate about that, too.
People lost their mind when Google did something like that in their image search, and unlike Apple, they provided attribution and a prime backlink. If Apple likes the service so much, they could just have bought it entirely and it wouldn't even show up in their balance, so proper attribution, and maybe ask the provider if he/she can handle the additional load, or even provide some support to handle the load, would just have been fair, don't you think?