It is not so much a security issue but rather when they encompassed DTD into the security bubble there are now limited number of checkpoints for GCH guests to enter through security. The prohibition of using the front entrance is so that other non GCH park guests do not use the GCH lobby as a shortcut into the parks backing up the security lines for actual GCH paying guests. That is why there is only a restriction on the front door and not the DCA or DTD entrances.
So...it's a security issue.
