The question would be why wouldn't Google keep their algorithm a secret? I know if I were Google I would do the same thing, as would everyone else.
If people knew the algorithm then the entire search result could easily be gamed, which could make the results completely useless. It isn't misleading a website owner, as the traditional method of building quality content that people want to link to still applies, and will always apply.
While it seems that Google controls all the Web traffic, you must remember that the online community chose to make Google the number one search engine. Back when Google started, sites like Yahoo were the top search engine, but Google produced a better product and eventually became number 1.