This is a mechanism based on the use of authority to censor, rather than speech against speech which doesn’t require authority.
Authority to move topics to a subforum should be considered strictly less than authority to ban/delete a topic outright.
The principle of least authority, which is usually for computer security but applicable here too, is that the least authority possible for any task should be used. Because it results in the least damage when misapplied.
Any justification based just on the existence of two sides is extremely vulnerable to shifting overton windows… basically it’s not hard at all to establish a polar opposite to a reasonable position just by expressing support for a ridiculous position. Or to put it more surreally,
“Even a single edged sword is a double edged sword. On one side it cuts, on the other it kinda doesn’t”
It’s a fair point, I don’t think any of these questions are easy, and for sure everyone on the internet and civil society is grappling with this. I’m just expressing a view here i haven’t been able to articulate coherently yet.