Dozy
It's a perfectly rational design, because what it is doing is performing in exactly the same manner it does in Normal Law.
Nope Dozy. In Normal Law it stops trimming at the V alpha protect if I remember correctly, but in ALT 2 it keeps on cranking away if airspeed is invalid until it hits the stops (or nearly so) as in the case of AF447.
But why make ordinary airline pilots into test pilots? Going boldly where no test pilot has gone before.
It is a certification issue. Gotta prove it meets the stall characteristic requirements with full nose up trim or stop it from autotrimming there. And for BOAC's benefit, no need to stop the pilot from doing what he needs with the trim. It is assumed that the pilots will act rationally.