Why silence was imposed on all englishmen working in the colonies as part of the imperial machinery?

Orwell is here referring to his attitude as a policeman in Burma. While he feels opposed to the British Empire, he is still too young and uneducated to be able to articulate this opposition. Further, in his far flung post in Burma, he’s cut off from critical political discussion and, as he says, he must work out his questions in a state of intellectual isolation.