This isn't Valve's fault. All they stipulate is that DLC must be sold through Steam, not that it can't be sold through other means. This is only fair when Steam is providing exposure / advertising and the distribution backbone. This is especially important now that Valve is supporting free-to-play games.
The reason EA is pushing this is that through Origin they take 100% of the profits, while Steam will take a cut (about a third according to some rumours). Who wouldn't want that extra profit margin? However, their decision here is to screw over customers who value Steam and use it as their primary purchasing method. The problem here is that it could easily backfire and I sincerely hope that it does.
I am REALLY looking forward to Battlefield 3 but I will not buy it if it isn't available on Steam. I will not support EA trying once again to screw over gamers by bullying their way into the market. EA has already failed with their previous digital distribution attempts and the restrictive nature of Origin (disabling inactive accounts and removing DLC, etc) is NO DIFFERENT. There are so many other games I'm looking forward to (Skyrim, Far Cry 3, Dead Island, Deus Ex: Human Revolution, Metro: Last Light, etc) that I'm not concerned - I already have too many games on Steam to play through.
PS - I'm very disappointed to see people siding with EA on this matter. Valve's policy is completely reasonable and I'm actually glad that they have the clout to stand up to EA.
"The price of freedom is eternal vigilance."