What part of the chapter was protecting Sasuke? If Itatchi went full out the fight would be closer but it's not like Kabuto is there to just kill them either. He said before the fight he'll leave Sasuke as he wants him for an experiment and he's trying to reclaim Itachi, notice he's mainly using finishers on Itatchi but ignoring Sasuke.
You do realize you are saying exactly what I am saying? They both are playing a game of who captures who first. Kabuto cannot kill Itachi because Itachi is immortal in this fight. So what Kabuto has to do to stop Itachi is regain control over him. Which means hes trying to recapture Itachi and capture Sasuke. Itachi and Sasuke are doing the same thing here. The only difference here is that Kabuto cannot take as much damage as Itachi because we do not know Kabuto's limits. So if Itachi and Sasuke were to go full out we know amateratsu would be the choice of usage since they are in a closed space.