They don't. This is a charge that never would have held up in court. It was included in a plea deal, so the defendant can admit to anything he wants.
This whole story makes me wonder if Flynn is actually the one who had all the leverage in this Mueller investigation.
Well, by lying to the FBI, he did discredit himself as a witness against CINC, no?
Flynn had $1M+ legal bills according to a report I heard today. He was out of money and had to take out loans on his house. Mueller had the upper hand in that regard...