He can't. But I can guarantee this: an employer can't force you to work more than you're willing to work. They can try but you're not their slave.
Employment is a two-way street.
If the employer isn't happy with your work, for whatever reason, they can fire you.
If you're not happy with your employer, for whatever reason, you can quit.
If an employer is making unreasonable demands on your time, you quit. And to be clear: there is no universal of what's reasonable, so I'm talking about your personal standard.
Employment is a risk for both parties. You risk that the job will be boring, or demanding more time than you're willing to give but you're not the only one taking the risk. Employer risks that you'll be unable to do the work or slack off etc.
What you say is true, but it's important to realize that the nature of the risk is disproportionate. The programmer quitting his job has a lot more at stake than the company firing the programmer. For the programmer, it means a stressful job search, likely financial stress (even if s/he saved up a lot of money, it's not fun to be living on a finite supply), and all the feelings that go along with losing a job (wounded pride, fear of judgement from others, etc.)
For the employer, it means what? They now have payroll that's significantly lighter, and true, they're down a person, but realistically that just means they have to pressure their other employees to pick up the slack while they find someone new, and those programmers, not wanting to lose their jobs, probably do it.
Now, of course the important thing as a programmer is to have an attitude like yours, which mitigates the downsides of leaving a job. But not everyone has that attitude.
Employment is a two-way street.
If the employer isn't happy with your work, for whatever reason, they can fire you.
If you're not happy with your employer, for whatever reason, you can quit.
If an employer is making unreasonable demands on your time, you quit. And to be clear: there is no universal of what's reasonable, so I'm talking about your personal standard.
Employment is a risk for both parties. You risk that the job will be boring, or demanding more time than you're willing to give but you're not the only one taking the risk. Employer risks that you'll be unable to do the work or slack off etc.