That makes sense, so I suppose the explanation might be that these bystanders were also outsiders, but, as turned out, presumably too late, they merely were in the wrong place at the wrong time as you say and guilty by proximity.

My initial reading was that they were known to be bystanders at the time of execution, which seemed a bit extreme even in that context. But you're right, it's equally possible to read their being bystanders as posthumous exoneration, which is much more likely.

(FWIW, I've been to Montana, and as a middle-class European, it certainly felt by far the most foreign of the 15 or so states I've visited in the US.)