Given the rarity of these sponsorships I think you have to accept TCA/XLA can place all kinds of limits and still come out with a high number of candidates who exceed their requirements - and who ultimately go on to win a place on the scheme.
If they didn't set some kind of limit then they would quite simply be inundated with applications, and ultimately all there would be to choose between two top-class candidates might be their grades, in which case the one with A-level s would surely win anyway.
That aside, every other sponsorship seems to have an age limit which those of us over the age of 26 just have to put up with. To me, this is rough going, but the sponsoring airline is quite entitled to do so. In this case the limiting factor is A-level results instead of age. If you don't qualify with the A-level grades there are other schemes out there.