Not defending this, but some people intend to pick up the bag on their return, presumably as they are headed away from a trashcan and will return to one on the way back. They don't want to carry it the whole time.

They should just carry it the whole time, or return to the start then and there to drop it

Sometimes people walking dogs plan on walking past the same spot on the return trip, so they leave the bag. Sometimes they forget to pick it back up, or forget that they dropped it there and take a different route home. Sometimes bag number two is the next day, or some other person's bag. Generally, if someone's going to pull a shit and split, they're not bagging.

Don't waste your time trying to analyze the decision making of an animal.