I am not sure how to explain my feelings on this, but here goes:
The Pooh characters were very well developed before Disney got involved with them. The characters were developed and they had a certain "look" to them. Disney took that "look" and Disney-fied it into the characters most of us associate with today. Yes, in recent years Disney has made their own stories with these characters, but in the beginning they were based on the Milne concepts.
As for the movies (like Cinderella and Sleeping Beauty and other Fairy Tale based stories) I have an easier time accepting these as Disney because there was no uniform look to the characters or even to the story lines (in most cases). Disney took a central tale that had many variations and evolved it into a feature length movie by adding characters and creating a "world" for them to live in, so to speak. This was basically already done by Milne in his books, Disney just basically needed to draw his vision, they didn't need to create anything.
I agree with your argument to a point...however I think that their are shades of gray between the black and white. While the other stories were not soley created by Disney, I think Disney created enough story elaborations and ancillary characters to take most of the credit for their vision. I don't think this is the case with the Pooh characters.