(
imho)
Succumbing to temptation...
1. Encourage third party publishers to deliver one-night-stands. Adventures or enhancements of low page count, minimum content of publisher's IP, building only on Wizards' material (no competing product lines).
2. Discourage third party publishers from establishing lasting brand names.
In short, they want the people to provide the most expendable type of product, which also happens to deliver least profit -
IIRC, Ryan Dancey himself mentioned that adventures and niche expansions deliver smallest revenues.
Products which compete via introducing strong thematic elements, long products lines and alternative or modified rules are strongly discouraged.
regards,
Ruemere