Because the book is an advertisement for whatever products he describes therein ("so they sat at the Au Bon Pain™, sipping Mochafrappalattes™ silently, watching

Because the book is an advertisement for whatever products he describes therein ("so they sat at the Au Bon Pain™, sipping Mochafrappalattes™ silently, watching each other"), and the advertisers pay per thousand books sold (CPM!), so the author wants to write well.