Disclaimer: I was born in the US but grew up in Argentina. I have a pretty good grasp of the subject, but I'm obviously not free of bias.
"What could possibly be bad about a young party of people looking to introduce some liberal, progressive changes?" you ask. Well, the problem is that helping a party that caters to younger people (read: rich and middle class young people; the working class in Argentina doesn't care about getting pot or being able to ride the Subte back from the clubs) can be seen as anti-government. And to a certain extent, it is. I won't go into what I believe about the current government or the opposition, but having foreign money influence a very vocal sector of the population is definitely meddling with the country internal affairs.
At the end of the date, this is drop in the bucket compared to the amount of money channeled through the State department, but if you put all like efforts in the same bag then you have a very strong case to support the theories that the US government is actively trying to get Kirchner and her allies out of power. It's not as overt as a coup, but it's still bad.