Because teacher pension benefits are not negotiated locally, but instead are negotiated and assumed by the state. Therefore, unlike the firefighters and policemen, for example, who have a "Stamford" pension fund for their benefits, Stamford teachers receive their benefits from the state-run Teachers' Retirement System (TRS).
The way the pension benefit is structured, benefits are a function of final salary, so Stamford does exert some indirect influence on our teachers' pension benefits by both deciding how many teachers to employ, and what to pay them. Accordingly, Stamford taxpayers are paying for the benefits of teachers from Hartford, and Greenwich, and Norwalk, and so on (and they in turn pay for Stamford teacher benefits).
All other teacher benefits (i.e. salaries, healthcare for current and retired employees) are negotiated and paid for by the cities. Governor Malloy has attempted to ask cities pick up a portion of the tab for their teachers' pension benefits, but has gotten nowhere.
I think our teachers are getting a raw deal. TRS is already only ~50% funded (and shrinking), as compared to closer to 75-80% for the city-run firefighter and policemen pension plans. Remember, Stamford teachers neither pay into nor receive Social Security. So, they are non-diversified and entirely dependent on the state to make good on its pension promises to retirees.
I have friends in SPS who haven't even hit 30 yet. Do you trust that Connecticut will be solvent in 2050 when they retire, and the state will be able to pay the benefits our teachers have earned?