I know a couple of the people at Shopzilla, and they're not idiots. However, it should be borne in mind that Shopzilla belongs to EWScripps who aren't idiots either, but do have the money to prosecute a case for a very long time indeed if they are so inclined.
Anyway, I'd recommend the same thing: contact them and see if the law firm does, in fact, represent them and isn't just 'Prenda Law'ing.
Well, they've apologised, explained what they were trying to do and sent wine and chocolates over.
I've got sympathy for them here: spam sites linking to you is a serious problem. One day the spam site has (undeserved) link juice. The next, Google's ban hammer comes down and you get unfairly penalised.
Anyway, I'd recommend the same thing: contact them and see if the law firm does, in fact, represent them and isn't just 'Prenda Law'ing.