They remove from the back very easily. Few tricks are as follows:
Pop in freezer : The humidity within a freezer is generally very low, and with the cold temperature the icing will harden up very quickly.
Edge of a table: If the icing is starting to peel away as you bend the backing, use the edge of a table to help release it. An important thing to note is that you are trying to peel the backing away from the icing, not the other way round, since the icing is very delicate. Run the backing sheet back and forth over the tabletop corner and you should see it starting to loosen.