Living in the SF Bay Area, I have witnessed people pumping flood water into the sanitary sewer. There are some low-lying spots where the storm drains can fail to keep up with the storms, and people get desperate when their homes are flooding.
In peninsula there are separate storm drains in the streets. Better in theory! But they don’t permit a direct connection from your French drain’s sump pump into the storm system. So some people pump into their household sewer instead.