U.S. primary and secondary schools do not accept the constructivist school as dogma. Perhaps you have simply misunderstood the differences he was stressing.

For more information on this, you should probably look at what Seymour Papert has been doing in Maine lately.

What he said was "people learn best when they are actively engaged with a topic, have to actively problem solve, as we would put it 'construct meaning.'" That isn't controversial at all, which is why I suspect it's a rhetorical trick. Scratch the surface and ask him why he thinks teachers really don't believe in it (which they all claim to do) and I'm guessing he'll say they don't believe it because they don't accept some other ideas (the entire dogma of the constructivist school, perhaps?) which he thinks are consequences of believing in engaged, active learning.

In other words, he's saying Y follows from X, so if you don't accept Y then you don't believe X. It's the same rhetorical device as saying, "Nobody cares about civil liberties anymore [because George Bush got re-elected]," or "I guess nobody else here loves his country [because I'm the only one who went to the Tea Party rally last weekend]."

It's always easier to stick up for a vague but popular concept (such as liberty, love of country, or active, engaged learning) than to argue in favor of the controversial concrete policies you think follow from it.

Anecdotally, I'd concur with the author. Most of my education seemed irrelevant. I learned to program and understand math because I wanted to make computer games.