nmap is in /usr/bin because there are things that the user are allowed to do with nmap, and it is a user runable program. However, if you want to do some of the advanced things, one of which that I can think of is sending a SYN packet, listening for a SYN/ACK, and then not sending the ACK packet back (somehow it tricks some OS's / services so that they don't log that they were nmap'ed), then you need root privileges. I think almost all of the -s options require root privileges. The point being, though, that the user can do a limited number of things with nmap.
-Bryan