Can you explain how exactly it would add confusion?
You must be new to the Internet.
So, you're not going to take a stab at a serious answer then? You felt sarcasm was the best way to go here?
See, there are going to be players out there looking at those single/multiplayer prompts and thinking "Uh.. well, I'm starting off singleplayer but I might want my friend to join me later.. but I might not. Am I locking myself into a choice by choosing one of these now? I better go ask on the forums..". That's "confusion". It also addresses your "who cares" question.
Remove an option and you'll have the same number or more people asking "where is the multiplayer option?" / "where is the single-player option?"
No, because the setting is already available elsewhere:
Options > Multiplayer > Game Mode> Offline/Invite Only/Friends/Public
And if that's not enough, it's available again in-game from the connection menu on the upper right corner of the screen:
Seriously, how many single/multiplayer settings do you need? Two separate instances of it is not enough? You really feel it's critical to have three?
The fact is, there seems to be zero difference between "New Game > Singleplayer" and "New Game > Multiplayer". Regardless of which path you choose you end up with a game that can be played either way.
A new player, however, is not going to know that. To a new player, that singleplayer prompt might as well be saying
"Are you 100% certain you're never going to want to have a friend join your game at some point far in the future.. like ever? You're ready to lock the option out now permanently?", and that multiplayer prompt might as well be saying
"This will alter your game is some way that won't be obvious to you until it's too late" - despite the fact that both choices do exactly the same thing.
Confusing players with a pointless option that doesn't actually do anything is, well, pointless. It might as well say "New Game > Grilled Cheese / Fishing Twine / Rome in June" for all the difference that prompt makes.