Proprietary software is not immoral, it simply belongs to someone, that's
all. If you make it open source under certain licenses like GPL - it stops
belonging to anyone. There are no simple answers. Saying that proprietary
software is immoral is baseless.
If they signed their software off, nobody owes them anything at that point.
I wouldn't want to live in a society where I am obliged to sell my property
to other people when they suddenly want it back.