Re: Is this legal?
Well, yeah, that is an acceptable exception to anti-discrimination laws. You are refusing to consider them for the job because they can not meet the requirements of the job and not because of an immutable characteristic of the candidate (EG, race, religion, etc).
But, also, I am talking about the employment laws that are implemented at the federal level for all employment in the US, not the laws surrounding federal-level employment. There are exceptions in the anti-discrimination laws for employment in roles that are safety critical or are national-security sensitive.