However, it is clear that not doing the work assigned (even if that means working more than 40 hours, i.e., working during times outside or normal business hours) is sound justification for termination of employment, so generally folks prefer to go along with whatever docking of pay that they're subjected to rather than get fired for cause.
I am going to go back and change my post. What it should have read is that the law is silent on using paid time off on days when you leave early even if you have worked more than 40 hours that week.
You are correct that failure to work required hours would be grounds for termiation. If an employee finds themselves being illegally docked they can file a complaint with the state or federal dept of labor while continuing to work.