Airlines have been discriminating in favour of military pilots ever since the days airlines came into existence. The military has always been one of the favoured sources for recruiting experienced pilot applicants.
The discrimination is the same sort of discrimination that favours type rated and experienced applicants over non-type rated and inexperienced applicants. It is the same sort of discrimination that favours integrated full time courses for cadet applicants, over modular piece meal programmes for cadet applicants.
It is the same sort of discrimination that allows a company or an individual to recruit what they perceive as the best applicants for a particular role. Discrimination is positive, and at the basic level a survival necessity. It may be "unfair" on ocaissions, but is neither unlawful nor immoral.
There are forms of discrimination that are unlawful, but since there is no suggestion that these companies are excluding anybody by virtue of that persons race, gender, sexual orientation or age, that wouldn't appear to be the case here.