There needs to be a greater acceptance that it is ok for kids to be different. My kids' love for math really shouldn't feel like a threat, any more than your kid's soccer ability is a threat....
I think that the root of this problem is inequality in this country. Soccer skills aren't intimidating because everyone knows that very, very few highly gifted athletes will actually head to the Olympics or the major leagues. This means that few will make a living in sports outside of teaching PE.
Alternatively, kids who are cognitively gifted have an apparently endless list of choices: engineering, science, medicine, public health, economics, writing, etc. etc. etc. So every time some grade-skipped giftie notices something that no one else did or develops a math skill well ahead of his classmates, people are reminded that he was born with options they don't have.
Our society tells people that they need a college degree to survive, yet college is too hard for a lot of people and many others would probably prefer to skip the debt and just go to work at a decent job. Yet those jobs are getting scarcer, and there's so much
everyone-must-go-to-college pressure telling kids that they need the degree. Under these circumstances, I can see that mentioning giftedness or the frustrations of math class moving too slowly could make people feel like their children's faces were being rubbed in the dirt of failure.
I'm not saying that it's right to have to keep quiet about intelligence. I'm saying that the other stuff is wrong.