People who work in offices are frequently referred to as "wh

People who work in offices are frequently referred to as "white-collar workers' for the simple reason that they usually wear a collar and tie to go to work.坐办公室的之所以常常被称作“白领工人”,就是因为他们通常是穿着硬领白衬衫,系着领带去上班.
语法书上解释是 to refer to...as.把.称作.
而句子中是are frequently referred to as ,为什么to后面直接跟了as,to后的宾语就不要了吗?