The reason the sunroof shatters to pieces unlike a windshield that doesn't, is the windshield has safety glass with a thin clear film sandwiched between the outer and inner panels of tempered glass, which holds the glass in place when it shatters.
What I find perplexing is that none of the sunroofs have shattered when they are in the open position and the glass is inside the roof panel. If the glass panels are under stress from improper heat treating, then why haven't any of them exploded while in the open position? Considering the interior is vented, heat inside the cabin would not pressurize the cabin to the point where it would break the glass.
I do agree that they should not be breaking so easily, even if a rock hits them.