Earlier quoted context omitted.
Why would you use ownership data for disk quotas? That's crazy. It should be based on file location. Anything in the user's home directory comes out of their quota. I assume that's how it does work surely?
What if Alice leave her home directory permissions open? Mallory can then move a large file there and have it assigned to Alice's quota. I suppose it could work, but Mallory would then risk Alice blowing that file away (perhaps innocently, not even realizing it was Mallory's file).
If I leave my front door unlocked and you put a large cow in my house, it's taking up space in my house.