I think you are correct. The problem is that it is random where / if the foam will strike the shuttle. The last strike hit one of the most sensitive and fragile areas - the leading edge of the wing. I would not doubt that foam has been hitting all along over the decades, but that was the first time it caused damage to a critical / sensitive spot.

I wonder if NASA would be better off not worrying about falling foam and concentrate on some kind of sacrificial (burn up on landing) "bumper" to protect the leading edges of the thermal protection system. This way if something hits at any time the shuttle is up or going up, you are still somewhat protected. This could also answer a "what if" situation of something (space junk) hitting the shuttle while in orbit.