First off, calling it discrimination is a bit bloody melodramatic... But then, so is calling ShesGreat a sexist and racist... Simmer down Hour Builder - no need to attack the people who you've come to for help. Believe it or not, we don't make these policies.
Everyone has raised really valid reasons as to why this might be the case, and Hour Builder, without providing the in depth specifics that an exact answer to this question would require, I'd think you'd have gotten your answers!
Of course, wrong or right, it may just be that airlines policy. I double dare you to challenge that in the PRE-EMPLOYMENT stage, and if your wife even see's employment, I (and probably any other airline employee, or indeed employee of a large company with high interest from prospective employees) could predict the sort of treatment she would receive from fellow colleagues/immediate managers/upper management.
Best of luck nontheless!