Earlier today I saw this sign:
煙{けむり}の行方{ゆくえ}。本人{ほんにん}だけが、他人{たにん}事{ごと}だった。
The English is different enough that I think we can mostly ignore it for the purposes of this question. It helpfully conveys the message, but I'm wondering about the grammar of the Japanese, from which the provided English is quite different.
In particular, I'm wondering about the use of だけが
. だけ
is "just" or "only", and が
is "but", so my reading of this is "Other people's problem, but only/just the person themselves." That's not merely deliberately ungrammatical English to convey my confusion, it also seems to convey the opposite message than intended. It seems to be saying that the smoke is only just the smoker's problem, no one elses.
From the context and the English provided, I know it's supposed to say, "not just the smoker, it's other people's issue as well."
But I just don't see how だけが
means "not just", instead of "but only."
What am I not understanding about だけが
?