It's much, much older than the illuminati.  They just adopted it.  Our culture is filled with such symbols.  The swastika is an ancient eastern religious symbol of prosperity and good fortune.  Kipling used it for many years on the bindings of his books until it was 'defiled beyond redemption' by nazi usage.  George Washington, like many of our founding fathers, was a bigtime Freemason, and Masonic symbolism is all over our American landscape.  Context is important, and symbolic associations change.