It was a judgment call. The theory is that casting write-in votes is extremely rare. Vote statistics bear this out. Even if you write-in one candidate, it is even more unlikely that you are going to write-in a bunch of races. Better to prattle at those exceptions rather than all voters.
Should the write-in entry instructions provided at the outset on the audio ballot not include an explanation of how to accept or cancel write-in definition, rather than presenting this information only in the course of write-in definition?