Michigan doesn’t have open primaries, open primaries are unconstitutional and have been for over a decade. They have open registration on primary day. Whatever ballot you pick up, you become a member of that party.
Michigan doesn’t have an open primary as it is commonly understood. When you show up to a primary, they ask you what party you want a ballot for. If you say Republican, congratulations you’ve become a registered republican. Vice versa. However, if you are say, an elected Republican or a Republican party official, likely you will be challenged if you try to get a Democratic ballot and could be pursued if you actually succeeded.
However, I do not think he is going after the individual cases. He’s going after Kos for telling Democrats to vote as Republicans when they have no intention to change parties. Since Michigan law is written the way it is, he argues (and cites the laws accordingly) Kos can be prosecuted.
And that’s exactly why they are subject to the FEC. They self-identify their purpose as to elect Democrats. That makes them a political committee like any other. We can argue the merits of election law and there is plenty of debate to have there. However, right now the laws are what they are and they should apply to everyone equally.