JB,
It's similar to the policy for eyesight. Although the RAF will keep aircrew flying if their eyesight deteriorates in Service, they won't consider an ab-initio applicant with the same perscription.
Not surprisingly, when you've got the ability to pick and choose, you pick and choose! (Although my preference would be to accept the guys with specs or with historic asthma issues and balance it with a radical increase in the standard needed on the aptitude tests)