The "no seat" rule is just there to make the airlines happy because
they can't sell a full fare ticket for an infant (they can try but
won't get a lot of takers). Of course the airplane is only 1/2 the
issue, having a proper seat for them once you get there is the other
1/2. Living in California, our child seat/booster seat requirements can
be quiet strict. Unfortunately, each state has slightly different rules
on what age/weight kids must be for each type of seat (including which
direction the seat faces). Its confusing enough in California that we
have regular semenars/inspections offered by local law enforcement for
parents. We went to several of the inspections, the kids loved it
because they got cheap plastic toys.
I'm not sure if Canada has a standard set of rules or if each province
has its own rules.

-Robert