I don't know if any books would get "flooded" with over bets, but the point is this: If the books didn't have this rule, they would put themselves in a position where they could only lose. Everyone who was going to bet a total would, as a matter of procedure, not bet any unders when there was a chance that the game might not go the distance. Overs would be OK to play, since you can only win and not lose if the game is stopped.
If the forecast was for rain, I would say that the books would have the usual number of over bets (maybe a few more) and no under bets, if they had to pay off when the overs came in.
Anyway, I understand your point. But you know the books are never going to cut any slack for the player. I do understand their position, in this case.