You are correct, the bracket does change the way the spring reacts. However, keep in mind, the resultant force vector is still applied through the body at a single point (the front spring eye bolt).

I see what you're trying to say though. I believe you're thinking that effectivly, the point of rotation becomes the contact between the spring and the upper rear cal-track pin. The spring does effectively become shorter (thus stiffer), but just like a ladder bar, even if it didn't flex at all, the effective point is still the free rotation point (front bolt).

Remeber the front bolt is a pivot, so it can not impose any moment (torque). It can only impose the x and y forces...