As the article mentions, despite her donating through the existing channels, depending on what Hunter's using the money for (and if it was purchased for an amount that most would feel is excessive, even with the subjectivity of art), it creates problems as that could be a way to circumnavigate certain rules with regards to the nature of political donations and the limits/caps on them.
For instance, if you started your own campaign and the laws dictated I could only donate, let's say $20,000 max.
Buying a piece of art work from someone close to you for 10 times that amount (and that person, in turn, gives the money to several other people, who can then each make their own $20,000 donation as a result), that could make for a messy situation.
That's why the political ethics analyst in the article stated:
"Legally, you might not have a problem," he said. "But with the appearance, it's a lot to try and explain."
The article also makes reference to the fact that while art is subjective, it's relatively rare that someone with no track record as an artist is going to be able to move $1.3 million dollars worth of art so easily with ever having a museum show or gallery shows prior to that. Or more succinctly, it's very uncommon for someone to have that kind of fiscal success in art when just starting out in their artistic endeavors.