Of course, sommeliers are invited to play golf as well some I know have even been taken on private planes to Burgundy by their best customers. (No, I won't be naming names.) And both retailers and sommeliers have been known to accept importer- or distributor-sponsored trips to visit key producers, whose wines will presumably end up on wine lists and store shelves.
Any publisher ordistributor who chose not to sign the agreement is free to charge whatever it wants for ebooks, and any consumer is free not to buy from those who did sign.