On page 310 of The Sense of Style Pinker states:
The terminology and analyses in this book are based on The Cambridge
Grammar of the English Language (Huddleston & Pullum, 2002).
On page 1011 of The Cambridge Grammar of the English Language Huddleston introduces the lengthy analysis that leads him to designating as prepositions what in traditional grammar would be called conjunctions, among them although, while, after and because:
The analysis of content clauses presented in this chapter differs in
significant ways from that found in traditional grammar.
Co-author Pullum has written an entry for Language Log in which he explains the rationale for regarding because as a preposition. It includes the following extract:
So what should we say about because? Contrary to all the dictionaries,
it is a preposition. As its complement (the phrase that follows it to
complete the PP) it may take either a clause (as in the PP because he
holds ridiculous beliefs) or a PP with of as its head (as in the PP
because of our public universities). Some prepositions can occur with
no complement (as in We went in), some require an NP (as of does) some
require a clause (as although does), and some require a PP (like out
in those uses that do not involve exiting from delimited regions of
space: notice that They did it out of ignorance is grammatical but
They did it out ignorance is not).
So, in summary, Pinker is following the CGEL in listing although, except, if, before, after, because, for as prepositions.