There's a mentality among a hardcore minority that singleplayer is or should be an extended tutorial for multiplayer. Look through the posts of everyone opposed to autoblock and you will see "MP" or "multiplayer" somewhere in their posts.
This is the bottom line for those people. They don't want autoblock in singleplayer because it negates hours of their work learning directional blocking for multiplayer. Because singleplayer should be primarily about preparing players for multiplayer and in these peoples' minds, multiplayer is the core feature of the game.
Which is crazy, because M&B isn't famous for a competitive multiplayer scene (or even a particularly large one), it's famous for the expansive single player campaigns and absolutely insane single player mods.
But there is something around very small niche multiplayer scenes I've seen time and again - a ridiculous level of elitism. Yes, you might be one of the best 100 players in the world, but when there's only a few thousand active players, it's not that impressive - get over yourself.