Thanks, superfly - puzzling to me. In EU-land, where RVR is not 'available' EU-OPS gives us the option of 'converting' met vis to RVR (although there was always 'discussion' (never resolved) about whether loss of RVR barred a Cat I approach based on the wording in our OM).
I have to assume that the relevant Air regs do not allow this conversion. Again, in ideal conditions on a full instrument runway, in EU-land a met vis of 600m would suffice by day and 300m by night, so why 800m I know not.