Petrochemicals coming off your bike go into drains and from there into rivers. Petrochemical chain oil goes into your garden when you wash your bike, in the street when it rains and in local woodland when you ride in the wet.
This can cause harm to aquatic life. Petrochemicals take a long time to break down if at all.
You wouldn't pour engine oil down the drain, so why allow chain oil to go down the drain?