As stated in the article, ignorance of the law is no excuse for me or you. It shouldn't be an excuse for the police officer.
As a minimum, any officer that arrested one of these men illegally and confiscated their gun should be fired.
Also, since it appears from the article that these were all black men, where is the FBI with their investigation? It seems to me this may well be racial profiling and a systematic violation of their civil rights.