That's a well-known thing. Turbulence injuries come from being launched into the ceiling of the plane due to negative Gs. It's why the seatbelt is important - to restrain you to your seat.
I think you might be severely underestimating the forces involved, as well as confusing "strong" engineering for safe engineering.
A ceiling that buckles when a human slams into it at an acceleration of -4g or more is a ceiling that doesn't severely fracture their skull and possibly kill them instantly.
