The mistake is believing that "intransitive" is an invariant property of verbs.
Transitivity is a property of clauses, not of verbs. Many verbs cannot be used alone in a transitive clause, and therefore transitivizing prepositions are used to make them transitive. These prepositions don't even have their usual meaning -- they're just crutches used to produce a transitive clause. Sense verbs are a good example.
- Bill heard the sonata. (transitive)
- Bill listened for an hour. (intransitive)
- Bill listened to it for an hour. (transitive)
- Mary saw the painting. (transitive)
- Mary looked for an hour. (intransitive)
- Mary looked at it for an hour. (transitive)
Clearly the preposition to in the third sentence does not mean what to normally means.
Likewise, the preposition at in the last sentence is not a normal use. Both are governed by the verb.
When a verb is transitivized by adding a preposition, the transitive verb is the
V + P combination,
and it can be passivized like any transitive verb
- They looked at/listened to/thought about/argued over/made much of it for decades. (active)
- It was looked at/listened to/thought about/argued over/made much of for decades. (passive)