It's harder to channel there, indicating that the Dark One has influence over the effects of channeling, which would make it even less necessary to police what items the various Forsaken bought to SG. If a Forsaken tried to use that much Power, the Dark One could just turn whatever they were trying to do against the channeler as punishment.

Perhaps this was instituted after Forsaken possibly fought and killed their rivals who were coming to SG for an all-hands meeting. After a while, it would get annoying to have officers and commanders getting nuked before they reported in by competitors, even if you favor a survival of the fittest approach. It would certainly be much harder to prosecute a war if the generals were assassinating each other on the footsteps of the meeting room before or after they had received critical orders.