Problem with how alias forms display autoreply text

I have created an autoreply message for an email alias. The message has the following text:

"
The message set out below ...

Original Message:
To: $TO
From: $FROM
Date: $DATE
Subject: $SUBJECT

$BODY
"

There are some more words after "below", but that is not relevant.

It seems the script will not let me use "$FROM" in the body like this without also using $FROM as the "From: address for replies" to the autoresponse. It puts "$FROM" in the textbox for that setting on the Basic Mode page, and if I change it to something else it removes the "FROM: $FROM" line from the text of the autoresponse.

Is there some reason for this behaviour, other than a bug?