They probably looked at some metrics... Single UL users might account for the bulk of the big data users so they got a bump.
I'm not going to dump on them too hard, they are trying to do what they can to give the best value but still make money. The 4 unlimited plan is a crazy good deal, and they aren't pulling underhanded tricks like adding line access fees.
Prices go up, this is unavoidable but necessary. They still undercut the competition, often by significant amounts. But it won't always be peaches and cream.