With print publishers, they are holding an author's intellectual capital hostage for YEARS and must offer an advance to do so. If a multi-book contract has joint accounting, the author won't see a dime more than that advance (which is often in the very low thousands, like $1500) until all the books on that contract have released and earned out the advance.
E-publishers pay royalties within MONTHS of the book being delivered by the author, so they are not holding IP hostage. There is no reason for an e-publisher to pay an advance.
Even so, some e-publishers *do* pay advances. see
www.loose-id.com/prospective.aspx
Yet another instance where RWA understands *nothing* about the e-publishing industry.