I believe the bushings were left off for two reasons. One, they don't serve any structural purpose. The spacers used instead of lower bushings actually make the frame/body stiffer than if rubber were used. Two, we're talking Regals- the low end of the Buick line, as far as cars with frames. If you look at the catalog, you will see that the LeSabre, the Invicta, the Electra, etc, got cushions at all positions, to isolate noise and vibration. The Regals were not expected to be as smooth and quiet as the bigger cars, so substituting a solid space in place of a rubber cushion was an "acceptable" compromise between cost and ride. So, the "design" was for a rubber cushion clamped between body and frame, with a big washer/spacer under the frame. But some cars got cushions, and on some cars the spacer was left out. Those cars would have been less rigid than the ones with the spacers, or the ones with the lower bushing/cushion.
But only the GM folks who made the decision can actually tell us what the thinking was.