There are no grounds on which to claim that these athletes have any sort of moral duty towards the society which raised him, as the society itself benefited from its investment and a moral duty should not arise from the accident of being born into a particular country.  The moral obligation, if it ever existed, is to the club and is fully fulfilled whether they stay at the club they were raised by or if they leave. If they stay, they will help the club win matches, championships and therefore money, which could in turn be used for the development for other young, talented players. If they leave, the club will receive a significant fee for the transfer, money which could again be used for the same purposes. Either way, they will bring significant advantages to the society that raised them, without having to play for the national team.