It has nothing to do with regulations, actually. These companies they work at aren't normally obliged to pay overtime by any regulation or contractual obligation. The engineers there are "exempt" employees, classified identically to what a software engineer would be.
In some cases these employers' contracts require them to pay overtime to employees that work on the contract project, but generally that isn't the case. They pay overtime because of the historical evolution of the industry and the expectations and dynamics of the employer/employee relationships.
