Why "uninterruptible sleep" is not counted as "IDLE"?

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http://unix.stackexchange.com – When I run top -bi, where the parameter -i means "ignore idle processes", the resulted list(see below) of processes are either R or D, although there're S processes at that time. So, why uninterruptible sleep is not counted as "idle"? And, it seems that "load average" equals the number of R processes plus the number of D processes? top - 17:47:44 up 141 days, 14:29, 2 users, load average: 4.10, 4.18, 4.17 Tasks: 224 total, 1 running, 223 sleeping, 0 stopped, 0 zombie Cpu(s): 0.1%us, 0.4%sy, 0.0%ni, 87.2%id, 12.3%wa, 0.0%hi, 0.0%si, 0.0%st Mem: 16432240k total, 16345816k used, (HowTos)