In contrast to Vtてある presented in Lesson 22-5, Viている simply indicates that something has happened to N, and N is currently in that state. It does not imply the presence of someone who performed the action. The verb used here(Vi) is a non-volitional verb(e.g., 消える[go off], あく[open/be opened], できる[be completed]).
The door is open.［This sentence simply describes the fact that the door is open.]
The door is open [because somebody opened it].
The sky is clear.