NCVIA was passed because of lawsuits over the Pertussis vaccine that were so extensive it threatened the entire vaccine industry. Rather than go back to the industry and say that you need to show that the benefits of the vaccine outweigh the risks with actual data, and not industry funded models, they just decided to hand liability to the taxpayer, through an excise tax. But the same taxpayer pays for healthcare too, don't they have a right to know if a treatment actually works, especially if it's required or requested to attend school/work?
"should be liable for their products" has an overarching implication that there's no liability. My own anecdata from the last few years is that the majority of anti-vax people parrot the line that "vaccine manufacturers are exempted from all liability"... which is patently false.