So getting the thread back on track to the title...one thought; They can't because they are the blinking directors and nearly always know something we don't which would be considered inappropriate/unfair advantage trading. (Trading on information not known by the rest of the market)
When you think about it, almost becomes a case of if the directors of any company are buying they know nothing more than the rest of us about the company they're meant to be leading, no market sensitive data?