First, let’s read what the Mueller report said about Don, Jr.
[The June 9 meeting] This series of events could implicate the federal election-law ban on contributions and donations by foreign nationals . . . Specifically, Goldstone passed along an offer purportedly from a Russian government official to provide “official documents and information” to the Trump campaign for the purposes of influencing the presidential election. Trump Jr. appears to have accepted that offer and to have arranged a meeting to receive those materials. Documentary evidence in the form of e-mail chains supports the inference that Kushner and Manafort were aware of that purpose and attended the June 9 meeting anticipating the receipt of helpful information to the Campaign from Russian sources.“. . .
There are reasonable arguments that the offered information would constitute a “thing of value” within the meaning of these provisions, but the Office determined that the government would not be likely to obtain and sustain a conviction for two other reasons: first, the Office did not obtain admissible evidence likely to meet the government’s burden to prove beyond a reasonable doubt that these individuals acted “willfully,” i.e. with general knowledge of the illegality of their conduct; and, second, the government would likely encounter difficulty proving beyond a reasonable doubt that the value of the promised information exceeded the threshold for a criminal violation.
Now — read the red part again. Don, Jr., did not have a clue that what he was doing was illegal. Just like his father, too damn stupid.