You are able to open and see the content of the loot bag without using it if you press escape after opening it. It allows you to decide whether or not you should use it to craft or take the loot. This bug is (I believe) really not intended.

EDIT :
You actually can take one or more items out but you need to leave at least one item in the bag. That allows you to take whatever loot you want and still be able to use the bag to craft the next tier with it. FYI you can take an epic loot bag and loot everything (except one item). Then you take the bag to make 4 rare bags and take the loot. After that you craft it back to epic. You can keep repeating those step to dupe stuff.

After further testing it prevent the use of the bag in the crafting process.