and then you say "well it's on the Apple Store for just $9.99" ...
They respond with "But I wanted a printed copy", and you go find a publisher, make a deal, put the content in the publisher's format, go through review and editing, and they can get a print copy a year or two later. Or you say "Sure I can do that" and reformat all your content a bit and send it to www.lulu.com
and then figure out how to market your new book
Or they say "but I wanted it on my Kindle from Amazon" ... and you take your content and put it into a PDF and hit publish to eBook and you set up your website to market it, and get some adverts and start a viral campaign, and push it via twitter and G+ and maybe sell a dozen copies.
... and you could have done all those before and without iBooks Author ... and you still can after
you use iBooks Author
But Apple are betting you won't, because they just lowered the entry bar into the market so low, you won't need to.
They aren't stopping you publishing in all the ways you already can, just limiting your commercial options with their software.