Not that I'm defending the bashing of Catholicism in any way (I'm Catholic!), the reason this can be done, as far as I understand it, is because of your previous question about "open" or "closed" threads. There are two kinds of threads in the Religion Forum, one, where anything but personal attacks on another FReeper is permitted, and another, called "devotional" threads where no bashing of any kind is allowed. That is, if an anti-Catholic came into a Catholic Devotional thread and started talking like they do on the open threads, then they would be reprimanded and/or suspended/banned.
As far as I understand it, this system seems to be the only way to continue to have discussions (as free as possible) about different faiths, as well as have a place where one can, if one wishes, simply participate in devotional activities where no fighting is allowed. IOW, it's a compromise.
But the RM could and does explain it better. I think the RM's profile page has a detailed explanation of all these things too.
I’ve found the system on FR to be generally fair. I’ve had some exceptional occasions of dismay at some of the actions of the RM, but there’s absolutely nothing for those of us to fear from anything that can be posted here. Emotional reactions are easy to come by, but our Church doesn’t hang in the balance over someone’s personal interpretation of Scripture.
I generally enjoy these debates because it exercises the soul and solidifies my faith even more. The trick is not to take it personally or dish it out personally. I can be fairly acerbic, but it’s not directed at “so and so”, but what “so and so” has presented as the truth.