It's a protective measure, to make sure the questions get answered on track.
If you see how it works out without this rule, discussions can get really messy. Even within one denomination, people get off track.
One example, in the Charismatic section, there are people who believe/don't believe in Word of Faith, which is claiming the promises of God. Last year, the subforum probably had five or ten debates on the same subject always going... day after day, never finding resolution.
Imagine if someone entered the discussion who didn't have the premise of the Charismatic belief, or Christianity. The original questions would never get answered -- conversations would go in all sorts of directions.
The CF forum tried opening up the discussions to more people, but every change brings new problems. Ideally, I think the CF Grand Poobah wants people to respect the premise of whatever subforum they are in.
Applying this -- you can ask questions, or even disagree, but not present an opposing viewpoint as Ultimate Truth, or tear down others. Most people would say "I would never do that," but it happens daily, hourly, constantly on the forum. So making these restrictions establishes a little more respect and boundaries between people.
It's certainly not meant to disrespect or alienate -- it can just end up appearing that way.