Only in Cataclysm can you say ‘I just wanted to store my cooking oil in the toilet’ and not get laughed out of the room.
I’m not sure why toilets can’t hold anything else. I thought maybe that the water is spawned in through code and that the toilet isn’t actually a container. But looking at the furniture items, toilets have the “LIQUIDCONT” flag. I’d guess that it’s a one-way container, allowing you to access its contents but not replace them. Erm, I mean, you knew that, but I mean ‘maybe furniture can’t have liquid dropped ‘in’ it without just dropping them on the floor, but can have fluids placed safely there for retrieval through the code’
Or it’s the game’s way of telling you to start hoarding jerrycans.