So the reason they have not released a +cellular MBP is down to them not wanting to make royalty payments? What do they care? They just roll that payment into the +cellular price bump. It's not like they are dipping into their own billion dollar cash on hand balance.
Is it possible its related to data traffic with carriers?
Do you have a better reason? I think the price difference is probably too high to justify the feature. I don't see any technical reason why they wouldn't do this. But patent royalties and additional overhead related to device certification would definitely add some complexity and cost.