q1w2e3 saidSomebody please explain to me why this is not mail fraud and prosecutable under federal law.
It may be, but it will come too late (the election in 1 week). Even if some front groups eventually suffer a fine (which Koch will cover for them), the election results would not be overturned on this. In fact, election results are almost never invalidated in the US.
This is the strategy of the Republicans. Use fraud and dirty tricks to win the election first, and worry about minor penalties later. Much later, at the snail's pace at which such issues are handled in the courts.
And don't forget, the Wisconsin government is now controlled by the Republicans, with radical Governor Scott Walker taking calls and instructions directly from David Koch. So don't expect any action at the State level regarding this. After all, they're the ones doing it, you could hardly expect them to investigate or prosecute themselves over this at the State level for election law violations.
One can only hope the local Wisconsin news media gives this lots of coverage, and further anger voters against Republicans.