1. No spam. It's important to understand that you can't "send" a pass to somebody's Passbook. Passes will only go into a Passbook if the owner of that Passbook specifically finds and chooses it. That person must see the pass and add the pass. You can't "get somebody's address" from some directory and then mass-distribute passes.
2. Inherently Opt-In. With email marketing, there is a large set of rules for single-and-double opt-in, designed to prevent email spam. These rules are needed because email is an open and unprotected system (really, never designed for commercial solicitation) and so opt-in rules have been created as fix for the problem of spam. WIth Passbook, the system is opt-in from the beginning. Consumers must specifically decide to add passes, and by making that decision, they are opting into that pass. Of course the pass providers must provide an honest and clear description of that that pass is for -- so there is a balance between adding as many cool features as possible, while keeping it simple!
3. High integrity. To provide passes to Passbook, you have to be a registered Apple certificate holder, and the terms of service prevent people from providing deceptive pass content. So that keeps the system clean.
4. Anonymous. A merchant can't "see" any personal information about the consumer who adds their pass, unless the consumer agrees to provide that information in some other way (for example by filling out a registration form, being an existing customer, etc). The merchant might know that someone has downloaded a pass, and there is no flow of information back. That's great for the consumer -- they can add all the passes they want, knowing that it's strictly a one-way channel.
5. It's like talking to a shop clerk. The easy and anonymous nature of Passbook reminds us of walking into a retail store and talking to a clerk. You can get a lot of information about the product you are shopping for, let's say, a new dress, but you won't be forced to give away your email address to that shop clerk. The clerk is there to help *you* with product information, not to drill you for personal details. You can walk away any time, without any hard feelings, come back later, whatever you want. It's all about easy consumer choice. Friendly, breezy, and easy.
A Win-Win. Passbook is a win-win design for consumers and merchants alike -- an easy, lightweight way for consumers to get the information they want on the products they want, without giving away personal information, but also a low-cost way for merchants to build long-term relationships with their customers.