Well, the working surfaces of a frizzen - the face, the bottom of the pan cover, the tip where it rests against the spring - shouldn't be browned. That really only leaves the back of the frizzen and the arm that comes to rest on the frizzen spring that can be browned. With use the area around the frizzen bridle where the screw goes through will likely wear bright in time, as well, though whether is is visible would depend on the shape of the arm, I suspect.
If they didn't brown the frizzen at all, I suspect that they didn't want to go through the trouble of browning it and then polishing half of it bright again. Would look a bit odd with the rest of the lock browned, though. Did they polish the lock or leave it as is from the manufacturer?
Incidentally, since you seem to be interested in being HC/PC these days, I'm pretty sure that locks were polished bright and then case-hardened, not browned. Polishing steel helps it resist corrosion better. The dark color of surviving locks is due to age rather than deliberate surface treatment.