I know it's not the same at all and completely hypothetical..
..but imagine if we sentenced someone to life for murder and another country came in and said "We don't think that murder is a punishable offence and you shouldn't be able to sentence that person to life" - thinking about it in that way, you can see how their beliefs work, and how our social norms are completely different to theirs..
Although I do agree that it is a completely unnecessary way to punish somebody, horrific really. And the fact that the trial was not fair, I think, is a much bigger point to be made.