In article om,
"Robert M. Gary" wrote:

The "no seat" rule is just there to make the airlines happy because

It's a little more sophisticated than that.
Airline seat backs are not as rigid as auto seats. The problem with the
earlier safety seats was that they would not withstand a rear impact and
would fold in half, crushing the child.

I don't see how the design of the airline seat makes holding the child
in your lap easier. We're not talking about child seats in this part of
the thread, only why the FAA allows you to hold a child.

The OP inquired about Canadian requirements for child safety seats.
He noted that the US had changed requirements. I provided the reason for
the requirements change.
Ten years ago, I commuted for a year between Ohio and Puerto Rico with a
3 year old and a five year old. I ALWAYS purchased separate seats and
brought along the child safety seats for my children.
The flight crews (Delta and USAir) were always very helpful, carrying
the two seats while I carried the children and carryon from the plane to
the terminal.