In fact, other providers are only providing images of "unknown" types, ie. on Amazon, you may find back cover used as main cover art.
Also in future, some providers may start to provide types associated with images.
So the assumption all providers but CAA provide front images isn't a good idea.
Also i'm thinking about future features, like allowing user to set type and order on user-provided images (and eventually submit them).
Still work in progress anyways.