Why was Mary Reibey a convict in 1791?

Mary Reibey was convicted of horse-stealing in England. This is the crime for which she was transported. Chances are high that, because her parents had died, she was running away from home (she was believed to be living with her grandmother at the time), and so she just "borrowed" the horse to get away. However, because horses were such a valuable commodity, the crime was considered serious.