Why don't piezo buzzers require resistors?
The description in the article you linked to says:


  The buzzer supplied in the EduKit is an 'active' buzzer, which means that it only needs an electric current to make a noise.


Notice that it calls it an active buzzer. An active device generally takes care of the fiddly parts for you, like oscillating and limiting current (within voltage limits which should be in its datasheet).