timppu: Seeing as the missing DLCs for Sniper Elite V2 cost around 19€, maybe they are hoping many people, who got the base game for free, use money on the DLCs?
mistermumbles: As someone else has already pointed out, it's very probably to promote the third Sniper Elite game. I looked at the DLC for SE2 and none of them seem very worthwhile. *shrug*
Well, they are probably expecting some people to buy DLCs, otherwise they wouldn't make them in the first place and price them so high, and not offer them as freebies as well along with the base game.
Promoting SE3 is probably the other reason.
Anyway, I think it will become more commonplace at least for less successful and aging games to offer the base game quite cheap (in e.g. Humble Bundles) or even free, and then hope people will buy any missing DLCs to them and get profit through them. Kind of similar idea as with free-to-play games.