Re: What happens if someone from the past shows up...
Then yes. It would be unjust and unfair for the United States government to prosecute a foreign national from a newly-contacted culture, especially if the U.S. government had never explained its laws to the representatives of this new culture.
But that's not the case in Justice
Yar, evidently, has access to their records as she states their laws are common-sense things. If the Edo provided the data and Yar missed something, its not the fault of the Edo.
"If we're going to be damned, let's be damned for what we really are." - Jean-Luc Picard, "Encounter at Farpoint"