Because if natural selection, as proposed by Darwin, is the main mechanism of evolution – there may be other things, but it does look as though that’s the case – then we’ve stopped natural selection.The headline says that humans have stopped evolving. If that's what he really said then it's easy to prove him wrong by showing that there's a lot more to evolution than natural selection [Have Humans Stopped Evolving?].
We stopped natural selection as soon as we started being able to rear 95–99 per cent of our babies that are born.
We are the only species to have put a halt to natural selection, of its own free will, as it were.
But what about his specific claim that natural selection doesn't work on humans any more? I covered that in earlier posts but John Hawks has corrected him today [Humans are still evolving, and soon we'll know a lot more about it]. I love it when I agree with John Hawks! (That's me, visiting him in his lab in Madison, Wisconsin.)