According to webmaster tools, google doesn't see anything that's on 'add to cart' button as a keyword. I changed 'add to cart' button to say 'buy' few months ago on both product pages and category pages and yet google still won't see 'buy' as keyword (optimization - keyword content in webmaster tools). I changed it to 'buy now' again few weeks ago to see if it makes any difference and still nothing.
It does recognize much less used keywords, i.e. words that appear only few times on some information pages, but not this 'buy' repeated 100's times on the entire site. Does google simply doesn't like phrases like 'buy' or 'buy now' and ignore them when scanning page?