Who says people sitting in the exit row actually know how to operate the door?
They have to be "willing and able"; they are not obligated to prove that they are trained to operate the exit.
Apparently being physically able and reading the card in the backseat is prima facie for being capable.
Personally, I think in a real emergency a large fraction of people would panic and botch it, but that's another story.
Anyway, crew can already change passenger seating at will, no matter what. If there is no FAA rule saying that somebody should near the exit, there should be one. It makes good sense.
Relax, nobody is taking away any civil liberities