US negotiators, especially Lighthizer, must have known that their demands would be rejected. They are about as far away from fair as you can get without tearing up NAFTA.

So why did they table them?

RC may be planning on using this impasse as justification to opt out now, using the six month notice period as a de facto time limit on further negotiations. He may also concurrently try to get enabling legislation through Congress to end NAFTA. Such tactics will up the pressure but I doubt they will wring more concessions out of Canada and Mexico. The opposite is more likely.

If NAFTA falls this way it seems unlikely to me that Canada and Mexico will immediately engage in bilateral negotiations since the US will probably make similar demands. It could be years before new negotiations start.

One is left wondering what the US is trying to accomplish, other than maybe consciously alienating all its friends, allies and trading partners.