I'm having this discussion on SimHQ(with a lot more conservative americans who find this quite hilarious and think all pirates should be excuted...), and I'll give you the short version.

IMO pirates, especially those who did not kill or harm the crew apart from locking them away or so, are criminals and deserve a trial in a court like all the other criminals do as well.
Killing people just to be badass or because they broke the law is just wrong and does not serve justice at all. Justice is served in a court, no matter how much proof you have, we do not shoot every bank robber here either. Maybe they do in Russia but then Russia isn't paradise as, ironically, conservative Americans(and they're the ones who seem to like what the Russians did the most) like to point out.

Anyway, these people are mostly fishermen who resort to blackmailing and robbing, since when do these two crimes justify shooting/otherwise killing a person without a trial in any western country? Why should we think it's fine in this case?