I'm less surprised that they want to keep it quiet, than that they thought they could buy someone who can afford a $100,000 car for $1500. Why be so cheap? instead, they just pissed him off and now it's all over the internet.
Also, calling it "goodwill" is perverse, coupled with the gagging threats. If it truly was goodwill, and they were confident they weren't liable, they why bother silencing the owner, shouldn't it be heralded as excellent customer service? They can't have it both ways. Either they truly believe they are in the clear, and are being super generous so all the passive aggressive legalese is unnecessary , or they realise there is a high likelihood of liability on their behalf, in which case, don't offer a settlement that still leaves the owner substantially out of pocket. it's a dick move, however you view it.