This metaphor might be saved if you consider amount of (limited) public attention to be analogy to amplitude of sound. You can softly type in your niche forum all you want, but nobody will hear you if there is somebody else screaming into the twitter megaphone nearby.

But readers have a real choice and freedom on what to read, in that situation, what benefit does restrictions on writers bring?

Ah - the goal is not necessarily the choice / freedom of readers, but that of other writers who might be drowned out, intimidated, or otherwise coerced into silence.

When this happens across a large and popular enough cross-section of media, though, it could easily start to have a noticeable effect on readers.