The Dictator game control is perfectly valid as a control for the Ultimatum game, since they're both equally lab experiments.

But it's a good antidote to for anyone who'd concluded "hey, people [in this population] are pretty generous" from Dictator splits (unlike Ultimatum, the offer can't be refused, so a rational agent should take 100% for himself - and people do, when they don't think they're in a lab experiment).