Yahoo users can't go to court if they can't prove their rights were violated. Even with this news story there is no court-admissible evidence of collusion with the government.
Also, from the perspective of a civil suit against Yahoo, this program is likely somewhat legal unless it was specifically against their privacy policy and terms of use.
That doesn't mean the engineers knew they were performing unethical behavior. A better question is: why should anyone hire someone who aided unethical behavior?
