To ensure the U.S. government is serving the interests of Americans.
It also promotes accountability. Take the trolls who provoked both sides of a debate into hostility [1]. That's not healthy for our democracy. But the FEC will have limited success going after the instigators. What they can do is tell American social media companies: "if you get paid to show advertising which, if on TV would have to carry a 'paid for by XYZ' disclaimer, then you have to get that information too."
Note that the FEC does not consider foreigners "who are...lawfully admitted for permanent residence" to be "foreign nationals."
Note further that nothing prevents e.g. the Russian embassy from taking out a full-page ad in a newspaper or buying advertisements promoting their views. They just have to say "this was paid for by the Russian Federation."
[1] http://www.businessinsider.com/russia-trolls-senate-intellig...
A little silly: Consider the examples. E.g., Russia took out facebook ads raising awareness about police brutality in the US. This is clearly "provoking both sides into hostility," but so what? Does it really matter that it's (scary voice) Russia doing it?
The second reason I think it's silly is it's just impossible to trace influence, ultimately. How many political commentators have gigs at think tanks that are ultimately funded by foreign governments? Lots! Or what about people that have close family members living in other countries? Etc.
Ultimately, I think people just need to evaluate arguments on the merits, and also be sophisticated about considering all possible sources and motivations for any given argument they are seeing.
You're highlighting the not-wrong parts of the illegal stuff and will next highlight the not-illegal parts of the wrong stuff. CA did things that were wrong and illegal, but not everything they did was both.
"Cambridge Analytica illegally deployed foreign workers to American campaigns"
Why is it (or should it be) wrong for a non-US-citizen to advise members of a US campaign?