When a block with NBT tags is placed, it actually loses the tags. I've attached a tag to a redstone lamp saying that it can only be placed on obsidian. This tag WORKS, it won't place on any block but obsidian. However, I've mined the block with a pickaxe (set with a tag that can break redstone lamps) and I got the lamp back, but since it lost it's CanPlaceOn tag, it can't be placed anymore.