Rubbish bag on bike

While parking my bike, I saw another bike with a bag of rubbish on its. Probably another person like me, I thought. He would have hated throwing the bag on the road, but couldn't find a bin and eventually decided to leave it on the bike - to be discarded later or in the hope that a curious person (thief?) would check the bag and decide to throw it away (probably to the road?) after finding that it's a bag full of rubbish.