Sitemaps provide a way for webmasters to tell search engines how to crawl and index websites more intelligently. Obviously it is a huge win for search engines and webmasters if all agree to use the same protocol.
This is just a guess, but I imagine that agreeing to release the protocol under a CC license saved Google, Yahoo!, and Microsoft many hours of legal haggling over copyright in the protocol. It is not a guess that this decision allows anyone, e.g., non-incumbent search engines, to publish and extend the protocol, without asking for permission from the incumbents.
Adding to the commons is win-win for current and future players, large and small. Don’t forget that Creative Commons needs your support to continue providing and developing licenses and tools that create an environment where win-win solutions for large and small are not only available, but the obvious right thing to do. In this too, follow Google’s lead. 🙂