And why should Russia NOT be punished for violations of (international/national) law ?
Everybody should be punished for violations of international law. The problem here would be that the complainant, superpower USA, itself is never punished for similar breeches. That would probably be the defense of the Russian government (if it is conclusively proven that attempts were made, and that these attempts had significant success), and if they then don't back down, what do you do then? Nuke Moscow?
Don't be silly. There are many vehicles to 'punish' a state for violating international law.
Sanctions are most widely used, but other method too. You can expel some diplomats. Or freeze their assets. Or you can get the UN to agree on any other measure.
It seems that in the RussiaGate issue, sanctions are an appropriate response, and the US has acted accordingly.
Also, you say "superpower USA, itself is never punished for similar breeches.". Can you give an example of that (where the US violated international law) ?
Looking at the bigger picture, I think the real problem is the problem of denial.
Russia denies interfering in the elections, and as with any denial, that means that there will be a group of people with doubts. These people will vent these doubts and create a counter argument, which will result in discussions that go nowhere. In the end, the argument becomes polarized, and everyone digs into their position. After that, no discovered evidence makes any difference any more, since there is always some crooked way of looking at the facts that re-enforces ones opinion.
We have seen that in the climate change debate, we saw it when Russia annexed Crimea and got away with it, when Russia started a war in Eastern Ukraine and got away with it, we saw it ever so painfully when Russia shot down MH17 and got away with it, and we see it again in RussiaGate, when Russia interfered in the elections of 27 countries, and so far got away with it.
Denial is the REAL problem that is dividing people, and it lets the perpetrator get away with hideous crimes.
Putin is the master of denial. All his actions are guided by the principle of "plausible deniability". He understand how powerful denial can be, and where to use it. That way he can get away with ANYTHING, and divide his opponents in the process.
And THAT is the real problem.