There are a number of reasons for it, one they can constrain what employees do, but they can't constrain what 3rd parties do. Investors are bound by their term sheets, employees by their employment agreements, and these people who hold stock and are 'unbound' might cause trouble. (not that they will its just that if they do the company has a limited number of ways to respond)
They can't cause any trouble until the employee shares have been delivered, which would be at the same point that the employee could unload the stock via other means; until that point, all they have is a contract with the employee.
Perhaps we have different ideas of what "trouble" is :-). My experience is that 'qualified investor' can often be substituted for 'troublemaker' but it may just be coincidence.
