Look at it like this:
Wearing seatbelts it purely a safety issue. It's also a legal requirement in the UK to protect car passengers.
So why is immunisation not a legal requirement?
Likewise, drinking and driving is a criminal offence, due to the danger to the passengers and other drivers/people around you.
But deliberately choosing to let your child become a carrier of a totally preventable disease, infecting people around them (including those too young for immunisations), is totally fine? If someone doesn't vaccinate their child, then the child subsequently becomes gravely ill, why aren't the parents charged with neglect?
Makes me think of this article
ONION