How would it be just as bad? Surely the mapper has better details (and per-map details) about how difficult it is to cpture a flag. Some maps could be single-cap epic pushes, some might be designed as run-and-gun wars of attrition where it's easy to cap but you need ten.
It's the same pattern that seems to work pretty well when it comes to capture point times, payload speeds, or respawn rates. The server could override either of them--and woefully we see instaspawn servers--but the primary responsibility for setting those values appropriately is map-based.