It also appears to contravene international law, surely they should only have one passport.....
Not in the UK, as along as you can give satisfactory reasons. In my case, (had two passports for almost 36 years), I used to specify; "For travel between countries of religious/sectarian difference", where it is important to separate the stamps of countries that disagree with each other, have territorial disputes etc. Bahrain and Iran was the case in point for original issue but there are plenty of other scenarios that fit the bill. I would strongly recommend any pilots that travel to the third world to carry two passports if they can.