geometry

when flight is cancelled, why does AA allow free changes to depart from another city?

example:

I was near SF. I had planned to take Amtrak to LA, spend the day there, and then fly AA from LAX to ORD.

The LAX-ORD flight got cancelled due to weather.(incoming flight cancelled, so the outboung had no aircraft)

I called AA and asked if they could change my flight from SFO to ORD. They did and gave me a new flight on the SFO-ORD flight on the same day. Got a refund on my Amtrak ticket later.

So why would they allow such changes and what is the farthest they would allow? would they have agreed to change it to HNL-ORD or SEA-ORD? I'm just wondering what rules and policies they were following...

thanks!