I'm sure some people think that ACS is simply a school for "the gifted," that encourages open thought and creativity and send their kids there not realizing just how bad the indoctrination is.
It should be noted that, in my experience, a lot of Ithaca parents are simply unaware of what is happening or even afraid to take on the schools here. The City is so tied to the educational establishment through Cornell and so many parents work for Cornell, they don't want to say much.
Also, I think some parents who skew liberal--but not THIS liberal--don't want to appear to be "politically incorrect," and even if their heads tell them this is wrong, they refuse to believe it.