I can only speak for the U.S., but "required to actually do the work" has nothing to do with whether or not I'm legally allowed to set a requirement for a job.

There are protected classes/categories that can't be part of the employment decision, but outside of those, the employer is free to set whatever criteria they want for the position.

If I run a plumbing company, I could say "all of our plumbers must have a computer science degree," and I can't imagine there's any legal reason I couldn't do that. I'd certainly have a recruiting problem, but not a legal problem as far as I'm aware.