Saying that people should be open to your arguments privileges those ideas you yourself hold. How do you tell which arguments get such treatment?
I didn't say at all people should be open to my particular arguments, but to arguments in general.
If it's arguments in general,
* you can't hang up on salesmen,
* you can't kick 911 conspirators off your unrelated forum,
* you can't remove most trolls,
* you can't ban homeopathy threads from /r/science,
* etc.
In general, you lose the ability to moderate conversations in spaces you own, or decide what you spend your time doing. This is, to be honest, a pretty extreme idea if taken literally.
Or you could deem salesmen, 911 conspirators, trolls and homeopathy proponents as "crazy and confused", but who decides that? How is that any more unwanted than posting critical meta commentary to /r/fatpeoplehate?
I imagine a concrete implementation of this idea as a meme that people themselves would recognize this as a good rule and that they would consider it as a decision aid whether or not to participate in a certain group.
The loophole, then, is to define ideas the politically powerful find objectionable as abuse.
No it's not. It's meant to protect speech which the politically powerful find objectionable. It's not about protecting abuse, but protecting ideas. Rebellious and revolutionary ideas, for example.
This is all a canard anyway - Reddit is not the (US) government. They get to set the rules for communication in their playground. If you don't let them do that, then you are infringing on their freedom of expression. Reddit can only say 'no' on their own systems; they don't stop you spreading whatever disgusting, heinous, pointless, hateful, ignorant, or foolish speech in other forums.
If you want free speech the way you describe it, then you need to go to an unmoderated forum. Incidentally, on the front page of HN at the moment is a request for help in removing child porn from a web service, because the owner doesn't have enough resources to moderate it. If you ever wanted a clear example of where free speech is not meant to protect disgusting material, that's got to be it