Whoever is appointed as the US President has an impact on 'Rest of World'. So why shouldn't RoW have an input on the selection process?
Foreign companies buy ad space in the USA all the time to influence customer behaviour. An election isn't much different on principle.
Laws against treason and foreign manipulation of elections exist so that foreign nations don't conquer and kill us all. There is no doubt Russia and many other nations want to do that. I guess that's not a good enough reason for you? Do you have a self-preservation instinct? Do you want to live? There's a few really good reasons not to treat advertising for elections like advertising for toilet paper. It's literally a matter of life and death. Lying about how effective your toilet paper is, is not.
They're not saying "Russia wants you to vote for Trump" — they're doing things like sowing discontent and divisive rhetoric. The current propaganda efforts are absolutely fueling a lot of the divisiveness in the US right now. It's dangerous.
You think it would be a fair election if Xi Jinping could drop a trillion dollars in the US to advocate for his favorite candidate?