That old saying “Ignorance of the law is no excuse” only applies to certain people and/or crimes.
According to the Mueller Report, though the investigation was able to prove the Trump Tower meeting that Trump Jr., Manfort and Kushner attended to obtain “information of value” would have constituted a prosecutable violation of campaign-finance laws, the Special Counsel did not feel confident they could prove beyond a reasonable doubt Trump Jr., Manfort or Kushner knew its illegality.
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 able to obtain and sustain a conviction for two 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…
Mueller Report Volume 1, Page 186
The Trump Campaign knew the information was damaging to Clinton. They knew it was coming from Russia. It was a “thing of value” to them. Yet through either denial and/or ignorance of the law, they were not prosecuted.