I would imagine that this is similar to other ACL products in which the NSA has previously expressed interest, like SELinux. The "labeling" probably means setting permission levels.
And regardless, all of the filtering would have to occur at the application layer, meaning you'd have to wrap every get/scan to have it do the filtering for you. The Accumulo way also gets you some efficiency because it never even has to transfer the cells that get filtered by the permissions (or even fully read their content from disk, possibly).
Even though each cell isn't separately encrypted to get you true security at the cell level (which would destroy your performance, I'd guess), this seems like a huge win if you want to have permissions at the cell level.