As AC*G, I've had UA automatically protect me on later flights when they felt a connection appeared to be at risk (I disagreed with them, and easily made the original flight). I've also had situations where they refused to rebook me into positive space on a later flight when mine was canceled. So it goes both ways.

As SE on AC, I've requested to be protected, but it's always been under odd circumstances (maintenance on board the aircraft and I'm concerned the crew is going to time out, etc.). I've never been protected automatically by AC, but I'm not sure I've been in situations where it would have made sense. I either can easily make the connection, or I'm delayed 2 hours when I had booked a 45 minute connection.

And I think that's what it comes down to. It's not hard for them to do, or against policy, but in most circumstances where "protection" would come up, you've completely blown the connection, so there's no point holding a seat on your original flight.