Last week, I was scheduled to fly LHR-ORD-X (domestic US airport.)

The international segment was delayed, meaning my connection to X was likely impossible. AA Twitter offered a later flight to X on another carrier, using the word protected. They also said I would keep my seat on the scheduled flight, in case I made the connection.

However, when I checked in the GA at LHR said I had to make a choice - either take a chance and stick with my scheduled flight, or take the alternative offered.

I have never had to do that before (been very fortunate) but womdered if that was how it was supposed to be?