Sony has been aggressive in experimenting with re-selling its legacy content. From MAG to Starhawk to, most recently, Uncharted 3, the company has been open to selling games "piecemeal," with the single and multiplayer functionality sold separately. Microsoft is sort of copying that approach with today's release of Halo 3: ODST "Campaign Edition," available for $14.99 on Xbox Live Marketplace.
According to the Games on Demand listing, the Campaign Edition "does not include the Halo 3 competitive multiplayer functionality or maps." There's no separate multiplayer-only version of the game to purchase on XBLM, either. (Halo 3 is available on XBLM for $20, complete with single and multiplayer.) The CE does offer Firefight in addition to the campaign mode, however.
This isn't Microsoft's first time breaking up a retail game for digital re-release. Fable 2 was released as a five-episode adventure on Xbox Live back in 2009. Oddly, that's also when Halo 3: ODST was originally released at retail--further proof that Microsoft really doesn't care for day-one digital distribution one bit.