I must confess, I'm not an expert on law, it chops and changes all the time and I only know the basics. I mean, you mentioned Pacman which is of course a PD clone as you say. The thing is Pacman (the character) is copyrighted, so if anyone basis a game on Pacman, arent the infringing some rights?
You're right with this assumption. These game clones are not legal in a way. But nobodey seems to care about it. (except Nintendo and the Tetris maker) By offering these PD clones for downloading we are in "good faith" that the developer has asked the character-, game principle owner for a licence and/or tolerates these game clones.
If we were asked by Namco to remove a PacMan clone we would not hesitate to do so. But not in the case of IDSA who create auto-generated e-mails and decline any form of communication.
As for screenshots etc Mr Creosote has already commented correctly...