Either the labeling is faulty or the moniker is overly optimistic. In either case, you don't HAVE TO prune it. But the regimen you describe is fine. I cut mine even more harshly -- down to 6"-8".

This is the second year I have had my butterfly bush, and I am planning on transplanting it to another area of the yard. Should I still prune it, or wait until next year, after it is reestablished in its new location? (Is the main benefit of pruning just to improve the density of its shape, so it isn't so spindly?)

macnang..I'm going to be transplanting a couple of dozen Buddleia soon. They're one year old, from seed. I'll cut them down to 3-6" when I transplant them. This forces multiple branch growth, giving you a much fuller plant.

My bushes bloom on new growth, which is why I cut them. back every year. For sooner blooming, you can just not cut them as far back. Leave on a couple feet of branch, instead of only 6-8". Yours being 6ft, doesn't sound like a really small bush. Mine get larger after being in the ground a couple years.
For better bloom, keep cutting the spent flowers off. I try to do my bushes weekly, then there aren't so many to cut. Keeps the new ones coming on steadily all summer.
I do think the annual shearing of branches makes for huge root system. I went to dig one out and the root was enormous! Only a couple years old. However all the butterfly bushes really started growth very late that year, in early July. I believed that bush had died, so I wanted to replant with a new one, dug out the old roots. I would wait longer now, to see if bush was truly dead, dig it out in fall.

I'm not sure what your zone is in CO, but depending on how cold it gets your Buddleia can freeze back quite far. In colder zones it is considered a die-back shrub. Here in MN (Zone 4), mine freeze back to the base and get to about 5-6ft by the end of the summer. They grow very fast once it warms up and start blooming around late June.