I think what it comes down to is that people are scared that Medium is going to use their license to steal all your content. Their license is weird: It says it allows them to make derivative works with your content, but at the same time it says it allows you to retain ownership of your content. What does that mean? Nobody knows.
I think if Medium just clarified that one line in their license it would make everyone's fears go away.
It is standard CYA terms to keep them from getting sued by their users whenever they decide to change the site in such a way that displays your content in a different manner than what you expected.
Every service you've ever signed up for that accepts user-generated/submitted content has a clause functionally identical to this.
Of course, every time someone reads one of those clauses they think it implies all sorts of evil, because reading comprehension is not a strong suit of the internet.
I think if Medium just clarified that one line in their license it would make everyone's fears go away.