The hard line pivots with the caliper so it can center, without brake line bias tension.
Cheers,
How is the hard line free to pivot when the rear is attached to the fork-leg via the rubber bushing and fender brace?
In what world is hard line more flexible than braided SS? 
Pretty simple mechanical physics, really.
The capture grommet is a very loose connection which behaves as a pivot or fulcrum, and is well behind the actual caliper arm pivot. None of the arm deflective moments are very great at that point, so the arm and hard line both pivot freely, so as not to interfere with caliper centering.
It is the real world where a flexible line attached to the caliper body applies more side force to the caliper than the hard line properly routed. It's pretty clear that Honda engineers had more training and knowledge about this, than home brew modifiers.
You can demonstrate this to yourself. Hold a yardstick in your hand, and place a 1 pound object on the end of it. The force applied to your hand is three foot pounds. Put the one pound weight at the one foot mark away from your hand. Now you are only lifting 1 foot pound and you will find it much easier to hold the weight up. With the weight or force farther away, it takes more force to effect movement or deflection farther away, given the same force applied. If you have ever operated a simple lever, you have already experienced these physics.
The fulcrum when a biased or under tension brake line is attached directly to the caliper, is applied at the caliper fitting. SS brake lines, while flexible, do have a reflexive memory. Go ahead, bend one. It won't stay exactly at the new position you put it in, unless you fasten it there. In fact, its reflexive memory is far greater than the caliper seal retraction effort, and possibly the adjuster spring, depending on how the line is routed.
Just like the weight at the end of the arm was magnified at your hand (fulcrum) by moving the force farther away from the fulcrum, the hard line attachment distance from the caliper makes it easier to pivot at the grommet location. The Calipers retraction or movement force is magnified by the distance or lever arm of the hard line at the grommet/stay.
The analysis only applies to lateral movement of the caliper. The vertical and longitudinal axis is constrained pretty tightly by the caliper pivot arm.
It occurs to me that some may not understand the proper hard line installation procedure. With the hard line properly routed, the hard line length touches nothing but the end points, and the fitting at the fender stay would float within that stay centered in the wire loop without the grommet installed. All the grommet does is eliminate chafing under G loads while driving. If the line is not deform from the shape given by the factory, it will all stay in alignment when reinstalled. However, if an untrained mechanic bends the hard line out of ignorance, then of course the system can't work as designed.
As XS correctly pointed out, when the lines come under pressure they move as the forces distribute along the length of the line. The fender stay helps to keep that movement from affecting the caliper. However, during caliper actuation, these forces are really insignificant. It is the caliper release and drag prevention that the original design dealt with effectively.
Cheers,
