PE, I am not sure if this is the case. I believe that it has much more to do with how the airports/terminals have been able to handle the installation of the x-ray machines (which are about the size of mini-vans).

For a lot of airports, the only place that the machines will actually fit is in the lobbies. IIRC, the airlines are responsible for getting the bags to the TSA, so the easiest way (for the airlines) is just to have the passengers bring their own bags to the TSA. The reason that BB ended up in line for an hour is that the bags cannot be out of control of their owner, the airline, or the TSA. It sounds like LAX could use more security screeners.

In SFO, for example, the same United checkin desks are used for flights to LAX or DEN as YYC or YVR. When you checkin with UA, they take your bag behind the counter (like the 'old' days) and then a porter takes a whole cart of them accross the lobby to the TSA, who screen them and send them back. You do not have to present once the bag is handed over to the airline, whether flying to Canada or not.

In some other airports, there is room the baggage holdroom (downstairs) for the x-ray machines, so the lobby procedure shouldn't have changed since 11 September.