The first thing that comes to mind is that Nvidia could reserve this feature (3-/4-way SLI) probably for their new big-Pascal GPUs (1080Ti/Pascal-Titan). Just to have something extra here to lure people into upgrading from 1080 again.
Anyway, if this "2-way SLI only" for the GTX 1070/1080 stays, then benchmark vendors should also invalidate 3- and 4-way SLI scores from these GTX 1070/1080 cards or block usage of more than two of them as this is no longer a valid config anymore.
Anyway, if this "2-way SLI only" for the GTX 1070/1080 stays, then benchmark vendors should also invalidate 3- and 4-way SLI scores from these GTX 1070/1080 cards or block usage of more than two of them as this is no longer a valid config anymore.