Why is the character 中 listed as a 嵌套结构 (nested structure) and not a 单一结构 (single component)?
The word 'radical' is usually translated from 偏旁 or 部首. But 偏旁 and 部首 are a bit different. People long ago called the left part of a character 偏 and the right part 旁. But later till now 偏旁 refer to components of characters. Some of the components that indicate semantic meanings are called 形旁(semantic radical). Xu Shen(?AD58-?147), the author of the book Shuo Wen Jie Zi(《说文解字》), classified the characters that have the same semantic radical into one category(部). This radical is always placed at the first in the list, so it's called 部首(literally 'category head'). 部首 is always 偏旁 but 偏旁 is not always 部首. According to Xu Shen's classifications, 中 is in the category of 丨 and 丨 is the radical component of 中.

As to the single-component structure and the nested-component structure, it's another classification. There have been several different principles to define if a character is a single-component or not. Single component structure character is translated from 独体字 and nested component structure character 合体字.

According to the theory of 'the six categories of Chinese characters' or 'Chinese character classification'(六书), the ideographic(指事) and pictographs(象形) are single components, the compound ideographs(会意) and phono-semantic compounds(形声) are nested. 中 is an ideographic character so it's a single component structure character.

As the character style developed from 小篆 to 隶书, and further to 楷书 widely used today, the shapes changed. The traditional classification was a bit out of date. Su Peicheng(1935-), a modern Chinese linguist, defined 263 single component characters according to the defination made by the Chinese encyclopedic dictionary Ci Hai(《辞海》, literally diction ocean, edited from 1915 to 1928 and first published in 1936）. 中 is included in the category of single-component structure characters.

In 1997, State Language Commission of PRC published a standard on the Chinese character set, which defines 280 single component structure characters, with 中 included, 丨 excluded.