If there's a practical physical reason it won't fit in the 4.7" size, fine, I get that.
But if it's just a matter of cost, it's really weird to me that they wouldn't offer it as either a 4.7" "pro" model of some kind (for example, if the shiny black ones came with the camera, but cost $50 more and had no bottom-tier storage option as is already the case), or as a standard feature only on the max storage tier (which could then also cost $150-200 more than the one below instead of $100 more).
I mean, seriously, if you can charge more for a 4.7" version and get some fraction of users to pay it (I would, and I wouldn't even consider the phablet size), why would you not do that and instead try to push them toward the bigger model?
(For that matter, the math seems suspect to me--the 6/6+ and 6s/6s+ models have nearly the same price difference at each storage tier as the 7/7+ models, some fraction of which did before and still does go toward a larger screen and battery, yet this claim is that the second camera included on the 7+ is a very expensive part, which would theoretically cut measurably into the margin of the 7+ model. Since the difference is now $120 instead of $100, if the camera costs $30-40 more, and that would imply that the optical stabilization previously cost $10-20 more or that the margins are now lower on the more expensive phones.)