I know of one cargo carrier (B777F) who has banned lithium batteries from the main-deck, consigning them to lower-deck Class C compartments only. A rather prudent step, if you ask me.
In an ideal world these batteries should, perhaps, be subject to an outright ban. But if you do that you might as well turn the lights out for air cargo. Thus a more safe way must be found to transport these units, and Class C compartment only is one component of a risk mitigating strategy.