It is literally the rule. Some of the things you're talking about in your post (e.g., the defender dropping a shoulder) are covered by other exceptions even if they occur outside the blocking zone. There is definitely some leeway given here as I don't think the exact parameters of the blocking zone (3 yards longitudinally and 5 yards laterally in each direction) are easy to see and enforce. The rules have changed over the year (e.g., getting rid of low-block zone in favor of a general blocking zone). I know other organizations have specifically addressed shotgun snaps disintegrating the blocking zone over the years, but I'm not entirely sure the NCAA has ever expressly addressed it in a casebook. But yeah, when the block in the back in question occurs 5 yards away from the line of scrimmage well after the initial contact and the ball is 10 yards away from the line of scrimmage, it is a penalty by rule.
Rule 9
Section 3 Blocking, Use of Hands or Arms
ARTICLE 5 Blocking in the Back
A block in the back (other than against the ball carrier) is illegal (A R 9-3-3-I, VII and -IX; and A R 10-2-2-XII) Exceptions:
1 Offensive players who are on the line of scrimmage at the snap within the blocking zone (Rule 2-3-6) may legally block in the back in the blocking zone, subject to the following restrictions: (a) A player on the line of scrimmage within this blocking zone may not leave the zone, return and block in the back (b) The blocking zone disintegrates when the ball leaves the zone (Rule 2-3-6)
2 When a player turns their back to a potential blocker who has committed themselves in intent and direction or movement
3 When a player attempts to reach a runner or legally attempts to recover or catch a fumble, a backward pass, a kick or a touched forward pass, they may push an opponent in the back above the waist (Rule 9-1-5 Exception 3)
4 When the opponent turns their back to the blocker under Rule 9-3-3-a-1-(b)
5 When an eligible player behind the neutral zone pushes an opponent in the back above the waist to get to a forward pass (Rule 9-1-5 Exception 4)
Rule 2
Section 3 Blocking
ARTICLE 6 Blocking Zone
a. The blocking zone is a rectangle centered on the middle lineman of the offensive formation and extending five yards laterally and three yards longitudinally in each direction
b The blocking zone disintegrates when the ball leaves the zone;