Themoffster:
The situation you cite shows the difference in whether a structure is designed to withstand a pressure differential or not.
All commercial aircraft are now equipped with "blowout doors" in internal bulkheads and dividers. These doors are designed to equalize pressure quickly in the case where a compartment is depressurized.
In the event of the cargo door opening/failing, the loss of pressure may be too quick to allow equalization, so the internal structure (the main cabin floor) failed. That is because it was not built to withstand the millions of pounds of instantaneous load placed on it.
OTOH, the "pressure vessel" -- the structure enclosing the pressurized part of the airplane -- is designed to withstand the loads placed on it by the air pressure differential, as long as the pressure is introduced at a controlled rate.