Bit o' Advice: Maintain accurate time keeping. Record your actual hours worked (by law on a USG contract you MUST but employers illegally may tell you differently). Keep timesheets. Keep company memos and pay policies regarding overtime. Build your case.
Is it because the employer's don't know any better? Emmmm..... no. They have lawyers. They get it.
This all falls under a little term the US came up with called, "Human Trafficking". It pertains to work without compensation.
Contracting officers: practice what you preach. All these rhetorical conferences I attend (yawn) listening to discussions about ethics in USG contracting.... puh-leeeeeze. Where is the enforcement? Where are the audits? Is it all taken at face value?