Because, from the writer's point of view, it would be equally good publicity if the venue published the work and paid the writer too.
This dubious 'publicity' happens automatically when the piece hits the shelves. The venues aren't offering publicity instead of payment; they're simply offering no payment.
Go ahead and admit your profit margins are tiny and you can't afford to pay me much. Just don't try to convince me you're paying me with something I would have gotten anyway.