Yeah, I don't get it either, unless, they want to give the 4K/UHD disc an advantage which is what I think the industry as a whole is doing here.I don't get why they wouldn't just use the ATMOS mix on the 3D and standard discs and save the trouble and cost of a different codex. It is not like the ATMOS mix is incompatible with standard Blu-ray discs. Why not standardize the audio codex across all the BD formats and quit futzing around and acting like including it on 4K but not on other discs is some sort of incentive to upgrade.
It isn't any deal breaker for me but, to me, it doesn't make any economic sense to be including a different codex on two discs when it could just be standardized.