docking pay for late clock in

Dear MEL,

I have two questions. I am employed by a small company (employing approximately 30 people) and they have recently enacted a new late policy. The policy is as follows; If an employee is one minute late they will be docked 15 minutes of pay. For each additional 5 minutes after the first minute late the employee will be docked an additional 15 minutes pay. I would like to know if there is any law governing docking pay for late clock in. Is there a limit to how much pay an employer can dock an employee for clocking in late? I understand that it is not to the employers benefit to dock pay for salaried employees, I am enquiring about this rule for hourly employees.
My second question is in reference to hourly employees working off the clock. What regulations are there on this issue. Recently my coworker clocked out then stayed to help clean. I am curious if this is legal. Is this considered volunteer work? What liability may this cause to my employer if the employee should be injured during this "volunteer" work?

Thank You