Because fish = meat. They are both protein, and the game sees them as the same food group. If the game had chicken, it would be lumped into the generic “protein” food group as well.
OP- there is an issue with the way shelters are stocked right now. The grocers are supposed to be stocking shelters with the different foods and luxury goods that the housing type requires. They currently aren’t. which is why you see your grocers wandering-unable to work fairly often when they are done stocking the actual market. They are supposed to be stocking items in houses from the market- they aren’t. The upcoming patch/update is supposed to correct this and have the grocers delivering food and other items from the market to the various shelters.