Some of the blame for Khan being in Into Darkness falls on places like this.
Before anyone rage-replies, allow me to clarify for a moment.
Immediately after the 2009 film, message boards and forums and the like were abuzz with "Will Khan be in the next movie?", "Who will they get to play the new Khan?", "I hope they don't put Khan in the next film"
Positive or negative, it was a lot of "Khan, Khan, Khan"
Now, put yourself in the shoes of the studio analysts who's job it is to see what generates the most buzz within the fan communities after the first film. Are you going to see the name "Khan" mentioned so much and NOT tell this to your bosses in the executive offices? And when those same corporate suits see one of the most oft repeated mentions within these fan pages at the time was "Khan", are you going to make any decision OTHER than putting Khan in the movie?
From their perspective, there was literally no other option.
Now, let me be clear, as Spock said more than once in TOS "I did not say I agree with it, I said I understand it"
What I do not understand however was the decision to lie about it. The whole time before Into Darkness, any time anyone mentioned "Khan", Paramount would either play dumb and say "I don't know what you're talking about", or blatantly lie with "Khan is definitely not in this film". If they were just open with it from the get-go, I feel the backlash would have been far less severe.
Its one of the big problems with social media, so much information goes around that it becomes nearly impossible to keep things quiet without blatant mis-direction, and it is extremely unfortunate. It makes things 10x more difficult than it needs to be, and itself creates its own problem. Back in the day, you had to deliberately look for the places that share information, theories, leaks and so fort, so it was far easier to avoid spoilers. Today, things just pop up front and center on your feed, and once you see them... you cannot unsee them.
Occasionally, without realizing, we become our own enemy in these cases.... and there is no real way that I can see to avoid it. Sometimes we just have to deal with the fact that we cannot just go back to the way things were, the genie is already out of the bottle. The only possible solution I can see is for everyone at large to stop caring about spoilers, then they will no longer have to swerve us... but that is not gonna happen.