Obsidian drops when I break ender chests with silk touch

When I use a silk touch pickaxe to break an ender chest, not only do I get the ender chest back, but I also get 8 obsidian. One would expect to only get the ender chest when broken with a silk touch pickaxe, and the obsidian when broken with a non-silk touch pickaxe, but for some reason I get both. This can easily be used to obtain large quantities of obsidian without much work.