Their right to believe
is just as valid as anyone else's. The problem is that various fundamentalists believe that their right to believe also covers being able to force others to go through the dictates of their religion, whether that means refusing blood for Jehova's Witnesses children, refusing medical treatment for "faith healers," or even having their specific beliefs presented in a science classroom. When secular laws try to prevent such people from forcing their beliefs onto others (yes, even their children in cases like this), they accuse secular society of violating
their rights.
It's just a misguided opinion on what the right to freely express your religion actually covers...and unfortunately in this case, kids can die from it.