Dani
The vertical stabilizer broke this way because it is designed that way. It is hold by several titanium bolts on top of the rear fuselage. The fin itself is designed much stronger than the bolts. So either the fin remains on the fuselage or it breaks. If it breaks, it breaks exactly there.
I think I understand the point you are trying to make, but you certainly have it the wrong way around.
The VS is held, as you say, by bolts to the rear fuelage. However, the bolts are far stronger than the VS, and (as in the case of the AA A300), the bolts remain in their mounting brackets, still attached to the fuselage, together with the broken ends of the VS lugs.
Yes, the VS tends to break at this point, but it is not "designed" to do so - it is simply the weakest point / link in the assembly.
HTH
FBW