I don't think this is a bug so much as an oversight, however:
In adventure mode, you can still take items out of item frames, put them back in, and/or destroy the frame entirely. This is even after recent changes to adventure mode that, by default, prevent the player from destroying blocks at all. The Item frame seems to have slipped through however, possibly due to the fact that it is actually an entity. But since it is used like a block for game purposes, it should probably be indestructible in adventure mode like actual blocks.