Why do taxpayer dollars pay for union business?

This morning, August 23, 2012, MSNBC reported that teachers unions spend 30 percent of the union dues they collect for member benefits. The other 70 percent is spent on influencing politicians. Now, I guess this also benefits its members. (I missed the total amount of money that the teachers unions collect in dues, but it was in the billions of dollars.)

HOWEVER, can someone tell me why in my school district, Rocky Point, as in many other districts, about 70 percent of the union president’s salary is paid for by taxpayer dollars?

And does anyone know why it is legal to use taxpayer dollars in this way?

Finally, if taxpayer dollars are being used to pay for union business, shouldn’t the public know specifically how that money is being used? Especially if the money is being used to lobby for a specific agenda?