This is a great question, thankfully with a deceptively straightforward answer!
This is an example of a stative passive construction. Stative passives describe a state rather than the result of an action (eventive passive.) Confusion arises because, in English, no distinction is made between the two types of passive construction. Other languages, such as Japanese, make this distinction clear.
There is a good article at Wikipedia (adjusted; references removed) which contains
Passive: Stative and adjectival uses
A type of clause that is similar or identical in form to the passive
clauses described above has the past participle used to denote not an
action, but a state being the result of an action. For example, the
sentence The window was broken may have two different meanings and
might be ambiguous:
- The window was broken, i.e. Someone or something broke the window. (action, event)
- The window was broken, i.e. The window was not intact. (resultant state)
The first sentence is an example of the canonical English passive [see
original article]. However the second case is distinct; such sentences
are not passive voice, because the participle is being used
adjectivally. Such constructs are sometimes called false passives
or stative passives (rarely called statal, static, or
resultative passives), since they represent either a state or a result. By contrast the canonical passives, representing an action or
event, may then be called dynamic or eventive passives.
The ambiguity in such sentences arises because the verb be is used
in English both as the passive auxiliary and as the ordinary copular
verb for linking to predicate adjectives. When get is used to form
the passive, there is no ambiguity: The window got broken cannot
have a stative meaning.
If a distinct adjective exists for the purpose of expressing the
state, then the past participle is less likely to be used for that
purpose; this is the case with the verb open and the adjective
open, so the sentence
in the stative case.
Past participles of transitive verbs can also be used as adjectives
(as in a broken doll), and the participles used in the
above-mentioned "stative" constructions are often considered to be
adjectival (in predicative use). Such constructions may then also be
called adjectival passives (although they are not normally
considered true passives). For example:
- She was relieved to find her car.
Here, relieved is an ordinary adjective, though it derives from the
past participle of relieve. In other sentences that same participle
may be used to form the true (dynamic) passive:
When the verb being put into the passive voice is a stative verb
anyway, the distinctions between uses of the past participle become
less clear, since the canonical passive already has a stative meaning.
- (For example: People know his identity → His identity is known.)
However it is sometimes possible to impart a dynamic meaning using
get as the auxiliary, as in get known with the meaning "become known".