Prime Day is here, and while Wal-Mart is copying the idea, I think only Amazon can make a national news story by unilaterally promoting a sale. And while good discounts on some Kindles, other miscellany, and fabled “lightning” deals every 10 minutes may generate some revenue, I have no doubt that the main thrust of the promotion is to drive new Prime memberships. After all, you can still join Prime and experience the fun with a free 30-day trial (or Amazon Student for 6 months free with a .edu address), and Amazon believes that once you go Prime, you never go back.