I'd say Winter Soldier lost 70% of the viewers when the movie "ended", and then was down less than 10% (having lost fist pump guy and almost losing Cap-t-shirt girl—but then they wondered why people were still sitting there) after the first post movie "bit".
Marvel could make them more up front spoken of—I'm sure a lot of people (outside LA—I think some of our 10% would have stayed anyway—I don't mind doing that as long as there's no parking lot snafu coming up) wouldn't stay all the way to the end unless there was nudity and gold. But apart from just putting them there, is Marvel keeping a cool club for us nerds by not saying anything? By putting in two, so there's an uber elite?
Theatres could tell people. Hell, I could stand up and yell "Don't you know there are post-credit scenes?"
Or maybe people just don't care.