Yes, i totally understand what you are saying, Michael. And yes, that would make sense to me as well.......but that's not how it works.
You need to assign the Environment Artifact Packages in the PrePlan step......the different packages for each environment.......still have to be assigned as part of the PrePlan.
The reason for this, as I understand it, is that a "Deployment Plan" object contains the artifact details required to create a "Deployment" object. In other words, the ONLY way to create a deployment with an artifact package (and it's ok to have a deployment WITHOUT a package) is from a DeploymentPlan that already has the artifact defined.
I'm probably doing a bad job describing this, so maybe one of the CA folks can jump in and make it more clear.