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.