The air dam was not bolted through the roof in model years 2000 through 2004.
If you wanted, you could unbolt the four hex screws securing the air dam to the rack, and remove the dam and store it in your garage -- and I did.

Nissan redesigned the way the air dam mounts on the roof for 2005 MY.

I wish you could remove the air dam for the 2005+ models.