Not so long ago, workers in the United States were subjected to long hours with very little pay, and they didn't have much choice in the matter if they wanted to feed their families. These days, the rules are different. In the state of California, employers must pay their employees at least a minimum wage, and they must also pay overtime if the employee works over a certain number of hours in one day, or over a certain number of hours in one week.
Some employers really don't want to pay that time-and-a-half for overtime, and they'll resort to some cheap tricks to try to get out of it. This may constitute wage theft, so make sure you watch out for things that violate your rights.