We lost a law suit once because a [not native English speaker] wrote "the bearing will fail with disastrous consequences", he meant the shaft would need to be replaced, but when a fire started in the area of the bearing that email was part of discovery and the jury decided the bearing was at fault for the fire not the user failing to clean the area which was our story. (legal would rather I not give you the case to read for yourself)

Chat tends to be less formal than email, thus more opportunity exists for something that is easy to take out of context. By not allowing saving chat logs we ensure they they will never get to court.

We have strict rules that emails must be deleted after a few months (unless we legally need to keep them). Any information in the email that is worth keeping longer than that is put in a formal system where legal can examine the words to ensure they won't be taken out of context and used against us.