with the assumption that the sleeve is necessary for protection of the disc...

My guess: It's an optimization. The sleeve tells them what disc is inside (to speed up inventory control), but that information is only considered trustworthy when the disc is sitting in their warehouse (since they'll make sure it's in the right envelope when they check it in). That way they don't have to pull the discs out of the sleeves and check them when they pull them off the shelf to send to you. No sense taking the disc out of the sleeve when checking it in AND when checking it out.