A vs de

A vs de
 

Hi,

...cuando no había sospechosos a los que torturar ni niños de los que
abusar.

Why a los, then de los? Is the use of a and de merely idiomatic? If not quite, is there some rule of thumb?

It's the way those verbs are built:
Torturar a alguien
Abusar de alguien/algo

Side note: I have seen "abusar a alguien" a couple of times, but this is because of the influence of English. It should be "de".