Alliance for Worker Freedom: AFL-CIO evading taxes

I've seen a few stories about the AFL-CIO using union dues to run ads against John McCain, and wondered a) why are compulsory union dues for explicit political activity legal? and b) how much are they spending?

According to their LM-2 (filed with the Department of Labor), they spent $42 million on politics. But according to their tax return with the IRS, they spent nothing. Why would they report differently to different federal agencies? It turns out that union expenditures on politics are taxable.

The Alliance for Worker Freedom estimates that the AFL-CIO owes $14 million in back taxes. They really are trying to emulate Al Capone.