I think that Helinut's response is probably the closest reason and that the "said company" has worded its conditions very poorly.
CRAZYBROADSWORD, is there no "let out" clause in the terms and conditions which would permit the employee to fly elsewhere ONLY with the express permission of the employer? This would be more normal and would assist the employer to keep tabs on the actual flying hours the employee is achieving. After all, your employer should have first call on your services - any other activities should be performed legally and should not conflict with the terms of your employment.