I read this in the Gifted Exchange Blog interview with Joel McIntosh, the publisher of Prufrock Press. I think it goes a long way in explaining the mindset you've encountered.
http://giftedexchange.blogspot.com/2009/03/facets-of-gifted-education-joel.html
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Unfortunately, there are many people who see the purpose of education as being nothing more than a guarantee that a set of agreed-upon skills should be mastered by every child. According to this kind of thinking, a gifted child who has already mastered these skills has little purpose in a school. In fact, this has been the driving philosophy behind the No Child Left Behind (NCLB) Act.

With this kind of mindset, it's easy to view parents of gifted children as just pushing their children ahead.

It helps me to think of a different mindset and something Tamara Fisher wrote about in Unwrapping the Gifted (December 10, 2008 Born? Made? Developed? Unstoppable?)
http://blogs.edweek.org/teachers/unwrapping_the_gifted/
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Why THIS random statement of his has stuck with me so clearly, I have no idea, but I've always remembered it... He said, "You're a band, not a point." ...the essence being that any test or measurement can give a good general idea about a learner's abilities or understanding of a concept, but on any other given day the person would test a little bit differently"
I take pride in stretching my children's bands but I recognize that each child's band is unique. Having gone through this as a child, I'm glad my mother did her best to make sure I was stretched and grateful she didn't do less because of other people's mindsets.