First, it is not illegal to "flop". Unlike the NCAA, nowhere in the NFHS Rule book is this addressed.
Next, the action of falling backwards is not in and of itself faking being fouled. In fact, in this situation the defender was indeed fouled by the player with the ball.
Finally, when officials employ their own philosophies when officiating, they end up making things up that are in direct contradiction to the written Rules, Official Interpretations and Points of Emphasis.....
Your philosophy does just that.
I already provided the written rule above, so I'll add not only the Official Interpretation but the Point of Emphasis related to guarding, both from the 2017-2018 season....
Official Interpretation
Play: A1 begins a drive to the basket. B2 steps into A1's path, has two feet on the court while facing A1, and just before A1 reaches B2, B2 starts to fall backward. As B2 is falling backward but has not yet fallen all the way to the court, A1 dribbles through B2's torso, which knocks B2 the rest of the way to the court. Since B2 was not completely upright when the contact occurred, who shall be assessed with the foul?
Ruling: When B2 had two feet on the court and was facing A1, B2 established legal guarding position on A1. After establishing legal guarding position, there is no provision that requires a defender to remain completely upright when the offensive player initiates the contact with the defender. Although it may be easier and would be more convincing to rule a player-control foul on Al had B2 remained completely upright when the contact occurred, a player-control foul shall still be assessed to A1 for charging into legal defender B2. Rule: 4-23-2,4-23-3,10-7-7, & 10-7-9
2017-2018 Point of Emphasis
Once a defensive player obtains legal guarding position by facing an opponent with both feet of the floor inbounds, he/she may move to maintain that position in any direction except toward the offensive player being guarded when contact occurs.
Please don't tell a coach that his/her player "can't start falling before the contact arrives". It's contrary to the rule and very likely contrary to what a good coach will teach their player to do.
You can have your opinion of the rule and you can certainly disagree with how it's written and interpreted, however you cannot have your own interpretation and application of the rule.
MSU
officiating is bad for the game.