I'd totally forgotten about that until you mentioned it, so I ran a Google and read the following Snopes article which gives all the details. Reading this makes the use of Zagnut bars in "Beetlejuice" seem all the funnier, because it's an extension of satire even to things like product endorsement. The producers of ET offered to plug one of the best-selling candies of the time and were turned down, so they ended up plugging a brand almost as popular. Sort of like a sports team trying to sign up one star and and not succeeding, so they end up signing someone almost as famous. In all these cases, the idea is to hook up with a "Big Name" to promote your enterprise. The Zagnut sort of reduces this idea to an absurdity -- pick the worst-selling candy you can find, and then have a character reject it and eat a fly instead.....
http://www.snopes.com/business/market/mandms.asp
