Who are the publishers who are telling you the author has to be the main marketing force behind a book? Honestly, I'm curious.
Why are so many books published without any meaningful marketing? Enormous amounts of editorial time and trouble are lavished on projects whose covers won't feature in print ads, whose placement in chain stores won't be paid for, whose authors and illustrators won't be signing copies for the crowds at conferences. One standard response is that there isn't money, but that answer won't quite do: there is money; it's just allocated other places. All those people who micro-tweak covers in meetings are getting paid, after all, as are the designers, authors, illustrators and printers for all that future pulp. Another answer authors hear frequently is that it's their job to market their books. No question, this is partly true. Certainly no one is more motivated. But just as certainly, no individual has a publisher's resources, stature or integration into the book world's network of communication and distribution. The Internet is a good tool, but there are still only 24 hours in a day and the author is still only one person. Aside from my personal stake in the issue, I've never seen the business sense in it from the publishers' point of view. Care to elucidate?
Let's first assume you're being publishing at a publisher that does allocate marketing money (i.e., not rinky-dink). You're right, most of that money is not going to go into print ads or conference visits or endcaps at B&N. And that's because those investments only pay off if they're based on a known reaction. They are not ways to create a reaction.
The marketing money for your book will instead be spent on making your book available and visible in ways that lots of authors seem to take for granted—the catalogs we print thousands of copies of, the websites we must maintain, the copies of the book we'll send out for free to reviewers and editors of magazines and awards committees, the trade and consumer shows where we will pay to have a booth, etc, etc. These are meaningful marketing efforts.
Perhaps you'd rather publishers spent less money on editorial and design work—less investment in making a book the best it can be—and more money on pushing it at the consumer.
Don't worry, I'm not about to launch into a "purity of the book" lecture. Editors and designers are in this business because they love books, but publishers are in this business to make money. So trust me, if it would sell more books to skip over the editorial and design side of things and just focus on shoving products down the public's throat, that's exactly what publishers would be doing.
Experience shows, however, that even on our own lists of carefully chosen and lovingly developed books, we can't always tell which books are going to get the reaction we're hoping for. And there's no building on a complete lack of interest from the market.
Once a book starts generating interest (if it generates interest), the marketing department will look for ways to encourage and nurture that interest, and it's at that point that you'll see the marketing pushes many lay people think of as actual "marketing."
Feel free to think of the publisher as a souless, mercenary behemoth that does not really care about your book. (Though this would obviously be a disservice to the underpaid, overworked, book-loving people who actually make books happen.) Just don't start thinking the publisher doesn't care about the tens of thousands of dollars it's invested to bring your book to market. It cares. It wants your book to earn back that investment. And it's doing what makes financial sense to help it sell, whether the author likes it or not.