I don't doubt there is from a Fifa and registration point of view, but the players get paid their July wages as severance if they are not retained, so just normal wages isn't worthwhile. If they are due to sign for another club the new club could block them too.I believe there are special dispensation rules in place for contracts due to expire mid tournament.
In reality it depends if the players really want to play in it or have the summer off before moving to warmer climates for a bonus year or two.