The answer seems to rely on whether or not there is evidence that fast food is inherently defective and whether or not the Big Food companies know it. The assertion is that it is not inherently defective and guilty knowledge would only be determined if a case went as far as the Discovery procedure.
Evidence from:
LS Pinchuk. Law.Com 28th Feb 2007
“Are Fast Food Lawsuits Likely to Be the Next ‘Big Tobacco’?”
There is, apparently, no evidence that fast food is inherently defective.
Further detail: