How about now. I think we all agree there’s a distinction between
Thread 1: “Declawing cats is an appropriate practice - only people who agree with this can post.”
and
Thread 2: “If someone wanted to declaw a cat, what is the most humane way to do it (NOT a debate on the merits of declawing cats)”
Thread 1 raises a controversial issue, asserts that one side of the issue is correct, and seeks to prohibit anyone who disagees from expressing their dissent. An OP is not allowed to do this.
Thread 2 assumes a premise on one side of a controversial issue, and seeks to discuss some consequent issue witout getting bogged down in debate of the controversial premise itself. This kind of restriction is allowed.
The problem with the “Positive Gun News” thread is that it is being moderated as though it were type 2, when it is really type 1. The thread allows people to post stories, implicitly advocating the view that these stories do constitute positive news, while barring people from expressing any dissenting view that the stories do not constitute positive news. Nothing consequent to an assumption that these stories are good news is being debated. The moderation simply allows the controversial view that these stories ARE all good news to be asserted without challenge.