De Wikipedia :
The name "niceness" originates from the idea that a process with a higher niceness value is "nicer" to other processes in the system, as it allows the other processes more CPU time.
Não seria bom se bons valores no Linux fossem trocados:
-20 = prioridade baixa
+20 = alta prioridade
$ renice -p +5 # ganharia mais tempo de CPU e
$ renice -p -5 # less
Por que foi selecionado para ser invertido?
De Wikipedia :
The name "niceness" originates from the idea that a process with a higher niceness value is "nicer" to other processes in the system, as it allows the other processes more CPU time.