Blockbuster has enough clout to force movie companies to "water down" their product so they don't have to sell "unrated / NC-17" editions of movies, why can't t

Blockbuster has enough clout to force movie companies to "water down" their product so they don't have to sell "unrated / NC-17" editions of movies, why can't they negotiate a much better public screening rate? Also, if it is so un-economical, why do the electronics stores that sell home theaters do exactly this?