I don't know if the Finder's get info works the same way as the chown command, but ONLY the owner of a file and the super-user can change the ownership of a file using chown. So this probably applies to your case (I noticed you had to do it as super-user).
When a file is copied, it is owned by the person who did the copying (ie, the person who created the new file). There are unix work arounds for this, but not in the Finder, that I know of.