mbcrowder wroteHmmm... food for thought, but the lack of the word MUST leads me to still see nothing untoward here.
The lack of the word "must" where?
Are you denying that a scrimmage down must start with a snap? Do you deny that the rules specify either what a snap is, or what must be done to snap the ball?
Think of it this way. Ball in player possession, player going down braces himself with the ball, sees a back behind him and tries to shuffle him the ball, but the ball ends up rolling on the ground...
Are you stopping the play? Calling that a bat? What? that can't be anything but a (bad) backward pass...
It makes a difference whether the ball is on the ground all the way from the time it leaves the player's possession, or travels thru the air any distance on leaving the player's hand(s). In the latter instance, it's a backwards pass. In the former, under the American codes, it's not a pass at all, and since they define "fumble" by exclusion, you'll see that's what it is, which means that NCAA's & NFL's rules on advancing a teammate's fumble come into play in some situations.
In Canadian football AFAIK sliding, rolling, or leaving the ball on the ground is a pass if it's intentional.