From a backpacker's forum:
In Seattle, authorities bought a list of every Seattle resident who regularly bought dog food. The authorities then compared this list to the list of people who had licensed a dog. If you regularly bought dog food but did not have a licensed dog, they sent you a nasty letter. At first, Safeway denied selling the information, but later fessed up.
I tried to verify this, but all I got was a ton of pages about dog food with no mention of customer lists. I didn't believe the first paragraph about Roomba, but it turned out to be mostly true.