For those who have missed the (many) pp, here is the explanation again:
You should not flush cat feces down the toilet. Studies are showing that toxoplasmosis, a parasite found in cat feces, has infected marine mammals and otters. Toxoplasmosis cannot be destroyed by sewage treatment, and therefore escapes into our waterways, where crabs and starfish will feed on it, thus jeopardizing the health of other marine mammals who feed on the crabs and starfish. Toxoplasmosis can cause brain damage and death in sea otters. Most (if not all) litter manufacturers warn on their packaging not to flush feces or litter down the toilet, and that is the reason.
Angie Bonnert, Sonoma Humane Society, www.sonomahumane.org.