The early 2005+ touring models did not have enough payload capacity (that is, GVWR minus empty weight) to hold eight people at 150 lbs each. In 2009 (I think), they bumped up the GVWR a bit, which allowed eight people at 150 lbs to fit within the weight limits. And that allowed them to add the eighth seat belt.
The non-touring vans were a bit lighter weight (less equipment), so they always had enough payload capacity for eight people.
If you search these forums enough, you'll find that there are people who have added an eighth seat belt to early touring models that never had it, but it's not an easy job. I believe it requires welding, but I'm not certain.
You won't get any help from Honda in adding a seat belt to vans that weren't originally equipped with one. They and their lawyers don't want the liability that would come from possibly exceeding the weight limits, or possibly installing a seat belt in an insecure way.
You can argue that the weight limits shouldn't be interpreted quite so rigidly and precisely, and that if you had eight people in the van, many of them would be lightweight children well below 150 lbs. And you'd probably be right. But the engineers and lawyers pay attention to the weight limits and have to draw the line somewhere, and that's where they drew it.