Schumer & Gillibrand have to bitch for optics, but most likely don't want the reason Berman was shit-canned to be revealed in hearings. I'm going with the theory that whatever Berman was doing to get shit-canned, Schumer & Gillibrand were involved ass deep in it.
Graham might have cut them a deal where they can bitch, but if they start fucking with the temp guy there will be hearings.
TL;DR Schumer already knows the answer to the question of why Berman got fired and doesn't want that information coming out in hearings. Graham obliges in return for a Schumer STFU in temp hire investigations. I.E., Schumer got an offer he couldn't refuse.