Theft isn't always a crime of premeditation, nor one motivated by financial gain.

Some people steal, just little things, just for the hell of it. You ever hear of someone shoplifting cigarette lighter even though they don't smoke? Same concept.

It's entirely possible that someone stole the bag just to be a ****.

And that's assuming a rational minded person. Some people have impulse control issues and there's always the posibility of kleptomania, a mental disorder where someone compulsively steals just... Whatever's on hand. Sometimes they don't reven realize they're doing it.