Please someone correct me if I'm wrong, but teams are not forced to let their players go on international duty, they can refuse and there's nothing national federations or FIFA can do... However, there is a sense of good intention by the club to let its players go to defend their countries if they want... It's kind an unwritten law...
For example, remember when Messi wanted to go to the olympics? Barca had all the right to forbid it, but decided to let Leo go because it was his wish to represent his nation... On the other hand, remember when Xavi (or was it Puyol?) who got mysteriously injured before international break and then played the next Barca game? It was obvious the player and the club made an agreement to "skip" international duty without making neither the player or the club look like assholes...
In other words, doing it outspokenly and against the player's will would be a dick move...