They're not conmen. They certainly don't lie in order to trick people and what they've researched is correct, maybe just one sided. Their motives aren't sinister, they just don't want anyone else to go through what they've been through and they'd be just as horrified if they convinced someone not to vaccinate and they lost their child to the disease.

I believe sometimes we can read too much into someone's motives, yet when it boils down to it they are really much simpler than we think.