We don't really know all of the details of this matter. Further, the only information we have if directly from the mother.
For all we know, she regularly rides the bus, and from time-to-time has a 5 ready to give the driver, knowing that the driver can't accept it.
Perhaps the woman who received a warning was a first time offender, and this woman was given a fine since she had a history of not paying for the ticket.
All in all, we can't make an assumption of the wrongdoing of either party involved, despite our initial inclination to feel sympathy for a mother with a young child.