(SPOILER)As qwerty has already mentioned, nobody knew that Master Xehanort planned to forge the χ-Blade. If anything, they were over-protective due to the state in which Master Xehanort left Ventus in Eraqus's care. When he arrived in the Land of Departure, Ventus's heart was only barely mended from coming into contact with Sora's newborn heart; they would have been careful of Ventus leaving because of his fragility.

That said, I do not think being confined to the Land of Departure was particularly uncommon; it does not seem that either Terra or Aqua had left before either; they were allowed to leave for the same reason that they were allowed to take the Mark of Mastery examination when Ventus was not: they were older, more experienced, and possessed more advanced training.