I dont think its about them being 'stupid'. I mean, not all of us know about the in's and out's of the law. An e-mail like that will strike fear into some webmasters and they could be thinking 'Is it really worth me doing this?".
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? It's the same with screenshots, isnt that technically illegal? In most terms and conditions it says 'in whole or in part' - Does that include screenshots or not? The law is a funny thing, its hard to understand and I guess it depends on how fussy the company in question is going to be. Like I say, I'm no expert, far from