True that it's ultimately the parent's call -- BUT, given the public health risks, if that's the case then the public school has the right to refuse to register the child. If it's on purely prudential grounds, you can't get a religious exemption (although I might well come up with my own homegrown Religion of Prudence, I suppose). And you can't put the kid in private school because they will demand the immunizations (and since usually there are 10 applicants for every place they don't have to deal with the annoyance). So home school is the only alternative if the schools refuse to accommodate ones concerns. If only one or two kids aren't immunized, they might well work with you -- but if there's a groundswell of anti-vaccination parents, there's going to be a problem.
I think taking out a truancy warrant against the parents is pretty extreme, it shouldn't come to that, but that's how a bureacracy thinks.
If enough kids aren’t immunized it creates a vector for an epidemic. However, if enough kids aren’t immunized it also creates an incentive for further research to more definitively answer the question of whether there is a correlation or not and to determine the causes of autism . . . which is an epidemic in and of itself.