Changing the selected (default) item in the panorama (which is what would be saved and restored when tombstoning) has the effect of moving the background relative to the items. It you have an image for the panorama background than this can be a big change.
I've known lots of people see the consequence of this and then decide they didn't want it. As such I made this opt in so you don't get unexpected and often unwanted behaviour as standard.
As it's VERY rare to have more than one panorama in an app I felt this wasn't adding a lot of overhead if you did want it.
I hope that clears it up.