> Why?
They don't have to respond to the demand letter, but they do have to respond if a lawsuit if one is filed. If they don't, it's a default judgement for the plaintiff. (This is one of the things that is broken in the American legal system. You can force someone to spend a lot of money by suing them even if your complaint has no merit.)
So while they don't have to respond to a credible threat of a lawsuit, it would be prudent for them to do so because otherwise there is a real risk for them of having it cost a lot more.