I think it's not so much the fact of influence, as it was the nature of it.
If, say, RT runs propaganda pieces in favor of candidate X, and against candidate Y, that's fine, because everybody knows that it is Russian propaganda. They can factor that knowledge into their decision making accordingly (and it doesn't need to be negative - e.g. someone who is interested in warmer relations with Russia, for whatever reasons, might actually support the candidates they endorse).
Coincidentally, this is also the kind of involvement that US and other Western countries normally practice overseas. E.g. looking back at the Ukrainian revolution in 2014, it was very obvious and transparent which side was backed by the US government.
In contrast, what we've seen in this past election is covert foreign propaganda, that tries to actively conceal and misrepresent its identity (many of those fake FB accounts and groups pretended to be American). Not only there are obvious ethical issues aside, but - in US, at least - it also runs afoul of the Foreign Agents Registration Act, which is a law that has been around for almost 80 years now. So you can hardly say that it's some new thing.
Then there's also the orthogonal aspect where a lot of that propaganda is blatantly false to an unusual degree. We're not talking about spin here, but actually manufacturing entire news stories around events that never happened, and facts that were never true.