"They are caught by police."
Translation:Mereka ditangkap polisi.
Halo teman! I think "oleh" can be optional with some verbs, when using the passive voice, and even some prepositions can be optional with the active voice for bitransitive verbs, for example when the two agents (primary and secondary objects) are recognized easily.
For example, the active verb (transitive) here would be "menangkap" (to catch, to arrest), and the bitransitive (named benefactive in the first Me-kan Verbs skill), "menangkapkan", meaning "to catch someone / something for someone else", which would use the preposition "untuk":
Example sentence from Kamus Besar Bahasa Indonesia (via Kamus Sabda - http://kamus.sabda.org/kamus/menangkap/)
Ia menangkapkan kupu-kupu untuk adiknya.
Possible translations: S/He catches / is catching / caught a / the butterfly / butterflies for his / her little sibling / sister / brother.
When the primary object is already mentioned, then it can be implied:
Ia menangkapkan untuk adiknya.
Or, replaced by a third-person substitution:
Ia menangkapkannya untuk adiknya.
When using the two agents and dropping "untuk", the sentence becomes ambiguous:
Ia menangkapkan kupu-kupu adiknya. (where "kupu-kupu adiknya" can mean "her / his little sibling's / sister's / brother's / butterfly / butterflies", which is / are caught for someone else)
Returning to the Duolingo sentence, I think the preposition "oleh" is often dropped in colloquial speech.
Since I am not a native speaker, I would like to hear other users' opinions about it. ;)
Please, do not forget to add your sentence with the reporting tool if it is rejected.
Selamat belajar! :)