because documentation is good. "hey, I changed your design and here's why XYZ" is far better than "merge me please". Also, he got to have fun writing it, and

because documentation is good. "hey, I changed your design and here's why XYZ" is far better than "merge me please". Also, he got to have fun writing it, and the bitly people got to have fun reading it, so it provides a PR bonus. :-)