I think DL figures a shopper on Google is weighing all options, whereas a shopper on DL.com is only looking at DL flights. Perfect opportunity to gouge a loyal DL flyer who books on DL!
Several DL posters who are more concerned with DLs stock price and thinning the herd would undoubtably give this policy an overwhelming thumbs up!
Corporate natural selection: only the savvy fliers get the full complement of available fares!