Downloadable content is always an interesting topic to read about. Each company seems to have a different philosophy on the matter. Harmonix has established Rock Band as a platform and uses the DLC to extend the life of the game (so they aren't releasing two sequels per year). Because they aren't planning to release sequels constantly, they are banking on the sale of DLC to generate revenue, therefore they need to charge for it, which is completely understandable. Valve, however, has a different idea in mind for Team Fortress 2 DLC.
Rather than banking on the revenue that could be gained by selling new maps and such, they have built the cost of producing the extra content into their budget. As such, they want to give the content away for free to keep the customers happy. If gamers know that they're going to get treated right and receive extra content after their purchase, you know that they'll be more than happy to buy another game from the same studio.
Unfortunately they will still have the hurdle of convincing Sony and Microsoft to release the DLC for free. I know in the past Microsoft has expressed that they are wanting all such DLC to sold, not given away.