Turbine, perhaps understandably, did little to promote this sale to current players who do not yet own the expansion - I only found out about it when DDO Cast re-tweeted a tweet about the sale. In fairness, the expansion has been around for a few months now, so most dedicated players probably snagged it at full price. Personally, I was almost certainly going to wait for a sale anyway, so it's not like I jumped on this instead of paying Turbine directly, though at least they would have gotten to keep the whole $25 through a sale on their own store.
Much like physical retail, the daily deal on steam does get a slot on the frontpage that all steam users see when they log in each day - I have no idea whether the studio provides Steam with a discount in exchange for this coveted placement, but it would not surprise me. Presumably, being discoverable by steam customers is the primary incentive for games going on the service in the first place, especially for online games that can generate ongoing revenue that cuts out the steam middle man.
The Secret World's investor update states that increasing distribution through channels that include Steam will somehow help their current predicament. They may not be the best exemplar, but most MMO studios not named Blizzard seem to think Steam's cut is worthwhile. They can't all be wrong, can they?