This point was raised when Gmail originally launched the feature. The problem is that it would essentially act as a read receipt, which is a privacy concern.
Plus it would add to confusion - you see it in your inbox (unread) and then you don't and wonder what happened. With Exchange you can do this and it usually just gets ugly in practice.
It would have to be based on any interaction the receiver has with the email, including viewing the inbox. However, if we're already worried about that level of privacy, even that change would do no good since it would mean the sender would know when the receiver checked their inbox.
