Blizzard screwed up. This is not an exploit in any way, shape, or form. It's like turning in your normal tier shoulder token at the vendor and having the vendor give you heroic shoulders instead.
Maybe wouldn't count as an exploit when the first person just uses the coin and sees the Heroic item. Might be an exploit when everyone else and their alts and their friends then go in and use as many coins on the encounter as possible.