Because the discussions are meant to be scientific in nature and positions should ideally have evidentiary support. These are not considered primary concerns in many of the faith forums. Don't ask me why, I don't frequent the faith forums generally either as a member or as an admin.
One reason might be that if the same rigorous standards were applied to the faith forums then a number of the creationist members excluded from the science forums for failing to support their arguments might similarly be excluded from the faith forums. I'm not sure if we currently have any such members active, but there have been a number over the last few years.
The existence of this 'double standard' has been gone over
ad nauseam in many threads both public and private. It still seems to be the best compromise which has been proposed.
TTFN,
WK