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It's an interesting concept, if you ponder it for a while. One of the reasons MLB can't get its plans for a "real" World Series (the "World Baseball Classic") off the ground is because the U.S. team owners (who have the power to prevent it) are unwilling to risk their multi-million dollar players in what are essentially exhibition matches. Also why the USA was demolished in the 2006 tournament, but I digress.

On the other hand, FIFA (a governing body that actually governs) requires each nation's football associations to release players to play on their respective national teams when asked, a requirement with roots in the idea that "exhibition" games between national teams serve to promote the sport.*

As a matter of contract law, both approaches are equally valid: the first, because the MLB player is essentially an independent contractor (and can thus be contractually-bound not to ride motorcycles, for example); and the second, because the FIFA player is a "temp," (and can thus be contractually-bound to appear elswhere because the team and the player entered negotiations with their "eyes wide open" regarding the requirement).

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*For an excellent discussion of the differences between Major League Baseball and Association Football, I recommend the book, National Pastime: how Americans play baseball and the rest of the world plays soccer, by Stefan Szymanski and Andrew Zimbalist (two economists writing for the squishy-Left Brookings Institution). It's a fascinating discussion of how the two sports evolved (simultaneously!), shared ideas in some cases, and went in different directions in others.