I think active political campaigning is a bit different from discussing political philosophy, and it's a major mistake to treat the former as "just free speech". (I think Citizens United was a massive misjudgement that has lead directly to many of our present-day problems). I think we're all agreed that foreigners should not be standing for office or voting in elections, and foreigners other than permanent residents are already barred from making campaign contributions; to my mind this kind of protest aimed at changing government policy falls into the same category. A protest like that isn't an effort to convey some insight or argument; it's an effort to demonstrate viscerally that the citizenry have a strong view on an issue. I don't think allowing foreign participation helps with that; quite the opposite.
There have been a lot of claims in recent years - from both sides of the aisle - that enemy countries have been deliberately disrupting US politics in order to harm the country. I think it's vital that our political process not only has integrity but is seen to have integrity, and part of that is ensuring that adversaries cannot unduly influence it.