Oh, that's in on_fs_type_changed, I was thinking it was in populate right side. The problem with putting it there is that it will get set, but if they select something else the label will stick, the user won't notice and we'll end up with a mis-labeled ext4 partition.
Doing it in save_right_side does it at the last moment, when we know for sure that's the fstype they actually want to use.