I agree that it is probably about the food. At the vet I worked for, we sometimes asked the cats to be left the night before, if we had ALOT of surgeries scheduled and wanted to start them first thing in the morning(6 am). My vet never kept any cats boy/girls unless they had a spay/neuter AND declaw at the same time overnight.
My vet for Stash is really strict too, and at first he was completely against me taking him on the same day, have 2 major surgeries and a spay, BUT I begged and called him about 20 times that day. Finally he said Stash could come home. I just kept bringing up the fact that they had NO observation after 7 pm, and if something were to happen overnight, noone would be there to know. If he was with me, that I could take action
I understand if the Vet disagrees with the 2 night stay, he just might want to make fully sure that his surgery went well.

PS- We always told everyone no food after 8pm the night before, but I let them eat all they could until 12 am, and extra canned food. They got spayed at 6 am, and as soon as I took them home, they pigged out on alot of canned food again