Let's play "Is it Magnetic?". The rules are pretty much self-explanatory.

Would you look at that, it's magnetic. We suspect that the charging coil's sticker backing works like a transformer core, made of up ferrous material that "serves to greatly reduce the magnetizing current and confine the flux to a path which closely couples the windings."

Tl;DR: we're thinking this sticker improves the inductive charging efficiency.

David clears things up for us:

"It is most likely a shielding for the main circuit board. When you are focusing a lot of energy through space, you cannot guarantee it will all end up on the receiver coil. Therefore you will need a shield to absorb any stray energy. Just think, if a wire just happens to be the perfect shape to receive energy, it will likely cook the circuit."