They can compete in offering a better product/service, instead of with a secret client list. Advertising itself is about connecting buyers and sellers - so maki

They can compete in offering a better product/service, instead of with a secret client list. Advertising itself is about connecting buyers and sellers - so making that information public would make the market all the more efficient! What's good for the goose and all that. Besides, the world is drowning in ads. Any downside for advertisers is an undiluted upside for society at large.