Because the GTX 970 needed 4GiB of RAM on paper to look appealing, but they couldn't allow it it to be as fast as the 980 or it would cannibalize too much of the more expensive card's market share. The partial disable "feature" is a feature for NVIDIA to enable them to produce more granular segmentation.
They could have disabled more SMs and just left the ROM/MC cluster fully enabled, but that would have resulted in the 760 ti's getting too close in shader performance and made the 970 slower than it is now, because it wasn't ROP limited in the first place.
The only other option would have been to disable the ROP/MC cluster entirely, which would have made the 970 a 3GiB card.
What NVIDIA did to the 970 was the best way they could satisfy the segmentation they want to enforce. Any other option would have resulted in either a slower card at the same price that would not have sold as well, or just a lower clocked 980 that would have sold too well, to the detriment of the more profitable part.
None of this excuses the distribution of false information, mistaken or otherwise, but it does explain why the GTX 970 is configured the way it is.