I have no doubt that you're right about the courts enforcing the contract - or in this case the lack of any language prohibiting an activity.
Many players have specific clauses in their contracts prohibiting certain activities. I've heard of players who aren't allowed to ski or even play pickup basketball due to the injury risk. IMO teams should contractually require players to be granted permission to play in the WBC.
If a player requested to play in the WBC and that permission was granted they may even consider that as an indication that maybe their future isn't as secure with the team as they wish it were - and reconsider the WBC.