I don't disagree with the fact that it's not a "bug", but I do believe this "functionality" needs tweaking.
First, the text above the additional plans refers to upgrading. Perhaps an additional language string can be added and referred to as "Additional Options" or something to that effect, since they are not yet part of a current plan, the term "upgrade" does not seem to fit here.
Secondly, based on the fact that this situation only shows plans that have the "allow upgrades to this plan" selected, it is obviously treating as or referring to the code related to upgrades. If they do not have a current plan and they're given options in case of a mistake or change-of-mind, then all plans should be shown (as "additional options"), regardless or whether or not they are plans which allow upgrading.
Let me know if you need screenshots or any more information about the above.
Thank you for all of your help, it is much appreciated.