It's an idiom that spreads through usage. As such, it doesn't need to be particularly meningful on its own; particles rarely are.
Imaginably, this usage is a corruption of "allow for [indirect object, dativ; whom?]", which is perhaps a little bit more natural ("to allow for the police to carry weapons"), which if coinciding with the direct object can be shortend either way ("to allow for the police", "to allow for weapons")--I mean, apparently that's what happened. After all, "für" in German commands accusative, "für wen oder was?" (for who or what) which marks the indirect object; in that sense, your question makes sense; but also cp Sp "por que", Fr "pour-quoi", Ger "wofür" (what for, why), and consider that English has for better or worse substituted, lost or fused cases.
"for" appears in a number of unusual idioms, e.g. the for-infinitive ("(for him to leave) is unimaginable to me", not for me!?). It's the case that particles like of aren't fully lexed. They are just there to fill syntactic positions. Generally, language learners just have to rote learn which prepositions, cases, infinitive constructions, and cetera may be commanded by which verb