The US justice system has a notion of "joint and several liability", where we say "both parties are guilty and both are liable for the damages". I think that applies here:
- It's the assembler's responsibility to make sure the exit door bolts are properly tightened, and
- It's the manufacturer's responsibility to find assemblers that correctly tighten door bolts, and the manufacturer's responsibility to design doors that don't explode, and
- It's the airline's responsibility to find manufacturers that design planes with not exploding doors, which are assembled by assemblers that correctly tighten door bolts, and checked by the airline's own maintenance teams during regular servicing of the planes.
Everyone should be looking over and double-checking everyone else's work. But that's not how the 737 MAX was designed or built.
- It's the assembler's responsibility to make sure the exit door bolts are properly tightened, and
- It's the manufacturer's responsibility to find assemblers that correctly tighten door bolts, and the manufacturer's responsibility to design doors that don't explode, and
- It's the airline's responsibility to find manufacturers that design planes with not exploding doors, which are assembled by assemblers that correctly tighten door bolts, and checked by the airline's own maintenance teams during regular servicing of the planes.
Everyone should be looking over and double-checking everyone else's work. But that's not how the 737 MAX was designed or built.