If you are salaried, your 40 hours a week is just the minimum. Some employers respect the 40 hours, some expect you to work well past that number until the job is done.
That does not mean they can order you around like a slave. What if IBM tells him that as an IBM employee he cannot eat at a particular restaurant, or eat pasta? What if IBM tells him that as an IBM employee he cannot sleep on a spring mattress? What if IBM tells him that as an IBM employee he cannot walk a dog on the weekend? What if IBM tells him that as an IBM employee he can invest in a mutual fund? What if IBM tells him that as an IBM employee he cannot help his neighbor in his garden? What if IBM tells him that as an IBM employee he cannot pee while standing? What if IBM tells him that as an IBM employee he cannot get a divorce? What if IBM tells him that as an IBM employee he cannot help his dad with a computer problem? What if IBM tells him that as an IBM employee he cannot answer a stack overflow query?
Unfortunately in at will employment states in the USA, a company could fire you for most of those things if they wanted to and it would be within their rights to do so. As long as they are not discriminating against a protected class they can fire you for almost anything.
I would like to see them try to make that stick in court with an example like, we don't allow our employees to eat chicken or they are fired.
The only place I've seen that is cash strapped and personnel short startups.