There is some truth to it; though I think part of Snowden's ability to publish the public-interest information is predicated on his threat to publish the really important stuff. Then again, he could easily just be a foreign operative, we never will know. I, for one, am happy that we have some knowledge of the programs exposed. I do not, however, harbour the illusion that Snowden is definitely altruistic.

Though to answer the "why not stay?" question; I don't honestly think that there is any practical way to expose U.S. secrets in the United States. By the time the news reports came out, he had already been targeted in several forcible operations. I also doubt that anything would have been done, considering that they haven't done much about it despite the entire world knowing.