If US embassy in Korea did anything similar to influence Korean election, and if it was found out, you can be assured that people will throw some major shit and there will be protests, both on the streets and in the national assembly, and heads will start rolling.
America understands this, and does not try to influence Korean election (or, if they still do, they are really careful not to get caught), because the risk of it backfiring is simply too much.
...And South Korea was basically America's puppet state during the majority of its existence.
>And still, Rule of Law requires that those who enforce the law, follow the law.
Countries that believe the US interferes with their elections are free to try to do what the US is doing, and attempt to prosecute a case against whomever they can catch.
Otherwise... what conclusion are we meant to reach? That countries which engage in espionage or which interfere with other countries' elections shouldn't have espionage laws or should allow any interference in their own? There's no argument of equivocation one could make to excuse or exonerate Russia in this case that doesn't also do the same for the US.
I could be wrong, but "Hacking an election" is not by definition illegal per say. However, accessing a computer illegally or defrauding people with the intention to meddle in an election is indeed illegal.
If <insert country here> had an election and the US-CIA had hackers put malicious code in their voting machines, and that country had laws against tampering with voting machines, then I would fully expect that country to indict those involved. Obviously in a geopolitical environment, that usually means through sanctions or some other means.
America understands this, and does not try to influence Korean election (or, if they still do, they are really careful not to get caught), because the risk of it backfiring is simply too much.
...And South Korea was basically America's puppet state during the majority of its existence.