Setting the difficulty of a mission is game content. Not being able to do so takes away options from the players, leaving them with less content to play. Same goes for strike playlists. While I get the nightfall being at level cap and the level cap being only available with TTK as well as the vendors not selling anything for you anymore, that just seems a bit too much to force players to buy the DLC. And it's not even necessary, because TTK seems very good so far!