Online servers have a protection system, so that when someone uses the copyrighted content, they are forced under the laws to preserve the integrity of the artist. Even if you own the file in iTunes, that doesn't give you the right based on their ToS to use it on the website. This happens on YouTube as well. They're more permissive but you will trigger a DMCA notice on your video, even if you can play it normally.
In certain situations (eg. Instagram), the developer will refuse to accept the content if it contains the copyrighted music.