An interesting comparison

Via the Gormogons, in an post on public teacher's unions:

Unions have but one function: improve their members' wages, benefits and work conditions at all costs.

Compare this to the statement:

Public companies have but one function: improve their investor's earnings.

Explain to me how the two statements are fundamentally different, and how we can hold one voluntary organization of individuals who are primarily concerned with making money to be better or worse than another voluntary organization of individuals who are primarily concerned with making money.