And when you do let the cat in out of pity, she'll start to cause chaos and you're helpless to do anything about it, for you have confined yourself to the toilet seat until the deed is done.

Same, but children.

Why Cats Follow You Into The Bathroom
My favourite reason is that they stand guard in case anybody tries to hassle you while you're pooping.

Who is going to rescue you from the evil shower if not Sam, uh?

Cats use urine and feces to mark territory. Your toilet is clearly your inner sanctum from the cats pov.

Sam just wants to be your Lord of the stool.