>Neither the name “Markdown” nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.
Gruber is not being irrational. He is operating under a different paradigm, which he explained last time Marco Arment was on his podcast. He is approaching this from the view that the optimal markdown implementation for a code site like GitHub is different to the optimal markdown for a commenting site like Reddit. Standardisation just locks in a suboptimal implementation for everyone. Whether you agree or disagree, it's at least a rational viewpoint.
The creator of the project let you do anything with it other than use the name markdown for a derivative product. They went ahead and did that. A completely unreasonable move that is just spiteful.