AI BOTANIST: "Captain, the AI crewmates may not need food, but our systems put a strain on life support. Some plants that can improve air quality will be replacing some food plants, which will unfortunately need to be many of them. So long as we resupply after a mission, food supplies should be sufficient...though it will increase overall mission expenses. Alternatively, Captain, if you decide having AI attending a certain position isn't worth it, you can request the engineer override our safety protocols and power us down. I will remind you Captain, I am available for consultation on the lunar atmosphere if desired."
(AIUI/OOC, and I must stress that I'm not privvy to the mechanics, it seems to be something to do with power for AI takes away from power for regular atmosphere scrubbers/reprocessors (thus needing plants to pick up the slack) and/or the growing of air-freshening plants (that aren't food-producing ones). There's probably some theoretical balance to this, but how set in stone or 'plot-driven' handwavium it is, I couldn't and wouldn't say.)
(i.e. think of it like ST:Voyager, only the holodeck/EMH is
not mysteriously powered by some strangely premise-driven incompatible supply of resources, such that though they may decide to ration replicated food they're happy for the Doctor to be on nearly constantly and as many interactive play-throughs of Captain Proton as your workshifts will allow. Here, instead, we've got the onboard servers trying to run three whole 'personalities' and that isn't cheap in processor cycles and thus power/cooling/etc.)