I don't have something I would consider to be a "valid" answer, but I'd like to blame the letter L - in lowercase form, it could be an I, an L, or the number 1.
edited to add:
On the UX side - my mind wants to make this a more valid answer, so I'm going to add that the real underlying intent (potential) could be that codes are a serious pain as it is. To have character confusion would just make it worse and insight anger, frustration, and lower customer happiness.
But that's just a best guess... I wouldn't be surprised if it was something more along the lines of the history of the label and its being printed by similar equipment as the postal service, which was only equipped to print or read capitalized letters, and that just being carried over into today's labels.