To this lot, "Abandonware" doesn't exist.
AFAIK "abandonware" doesn't exist in a legal sense either. It is true that some companies have gone bankrupt and left behind games for which noone seems to own the copyright - however, I don't believe this means the copyright is automatically dissolved. Copyright lasts for many years (75 in the US?) after the death of an author of a work.
Whether we agree with this or not, the fact is that the people you mention above are merely abiding by the law as is their right. As Matt said, you can't blame these guys for protecting themselves from legal action. I would also argue that - especially in the case of ScummVM - the attitude and stance of the team surely fosters a greater cooperation between developers and the community.