The direct translation to your sentence would be something like:
La sociedad desaprobó (...) —y ello contribuyó (...)— pero este motivo (...).
Instead of the current:
La sociedad desaprobó (...) y ello contribuyó (...). Sin embargo, este motivo (...).
However, the translator may have thought that splitting into two sentences makes the whole text more understandable. Both sound fine to me, since they respect the meaning and keep the focus on the same aspects.
You then ask:
is it reasonable to expect that English can always map to Spanish on a sentence-by-sentence basis or do linguistic differences mean that what can be expressed in one sentence in one language may take more in another?
This is difficult to say in a generic way. In many cases, it is true: you can express the same idea in a sentence.
However, it is important to note that Spanish sentences tend to be longer than those in English by about 1/5 to 1/4. This means that there may be an inclination to split those that become too long for Spanish, while they are still "short" in English. May this be because English idioms make the language more terse, I don't know.
As a side note: I once was translating my cv from Spanish into English and a girl who had a better knowledge of English than me said: Hey, fedorqui, just use short sentences. So I ended up splitting long Spanish sentences into smaller ones in English. But, still, I have the feeling that my English sentences are way longer than the ones native speakers write.