Maybe I should ask: "why do we require that the people who teach our children in public schools pay union dues?"
Don't get me wrong-- I think teachers should be free to join whatever organization they want.
But why do we REQUIRE union membership (or at least payment of union dues) to teach in the public schools?
Do we think teachers are too dumb to negotiate their own salary?
Do we think that school boards and principals and superintendents will trick teachers out of fair salaries and benefits?
Unions are supposed to improve working conditions and get better salaries and benefits for their members, and there is a "free rider" problem if membership is voluntary-- nothing but Massachusetts State Law stops somebody from deciding NOT to pay for a union that is negotiating for everybody.
So it is obvious why unions and unionized workers like laws that require employees to participate in collective bargaining agreements. They get better working conditions and higher salaries and benefits!
I can see the 'public good' angle for better working conditions; working conditions for people in slaughterhouses in 1910 really were terrible, and those terrible working conditions caused external costs that weren't paid by the slaughterhouses (like sick or injured workers that had reduced life expectancies). Working conditions for teachers in 2011 everywhere besides inner-city schools seem pretty good to me, though, and I doubt they'd be worse if the teachers' union disappeared.
I can't see any public good angle for higher-than-average wages and benefits. And if you disagree and think teachers are overworked and underpaid and have to put up with tyrannical bosses and bad working conditions... then tell me again why teachers are forced to pay union dues? Wouldn't they be better off just keeping all that union dues money for themselves?