OK, I think that's right and I believe you. I just asked this, because I remember an article. It also mentioned that the hardware is exactly the same.
It said that the lasers of the very same brand, model and sub-model have different productional quality and therefore different power resulting in different precision. The drives with parts that are able to burn DVD-RAM are labeled 7xxx/non-D and the others 5xxx/D. Maybe heavy DVD-RAM users who upflashed could report if their drives produced problematic DVD-RAM burns/died earlier than expected or not.
But I can't find the article anyway. I searched a lot to find it. Sometimes mind plays tricks on people. Maybe I remember wrong. So I guess you all are right with the licensing costs.