They are completly at fault, putting a disclaimer in that says this will end the game and then selling dlc that takes place before that point is ass backwards.
I don't understand why you and many others are saying it's the players fault Bethesda and Obsidian were fully aware this is how the dlc was going to work.
That said it should have force saved to give players the option for the future and not assume that players can see the future for the game and make additional saves "just in case".
Saying that Bethesda and Obsidian are not responsible is ignorant, I wouldn't expect anything of a player then what is given to them.
By making DLC like this they are making people do something that is not normal for a console gamer which is save more often then the game indicated is needed.
I still don't see how it's the developers fault.
FO3 had several DLC's and it was a given that New Vegas was going to have them as well, in fact, they said they were before the game was released IIRC.
So expecting the player to save before the final quest?
Well... Yeah?
That's why they had the pop up, to make sure people saved before hand.
If not for the DLC then for themselves.
In case the player wanted to do additional exploring or side quests or collection hunting.
And being an RPGer for 12 years I've learned to save a lot.
New RPGer's aren't screwed over at all, they flat out gets a pop up handed to them on a silver plate telling them they should probably make a save before entering the final quest.
And finally, do some research, not you specifically but others.
I NEVER buy a game, DLC or expansion before I know a greater deal of info about it.
If some people decide to just up and buy something without researching it first then again, their fault.
It's like ordering a pizza cause it's name sounds good and being mad when finding out it has mushrooms on it. (Which lets say you/I/someone don't like.)
If they did the research they'd know the DLC could not be played if you are inside the Dam.