I wouldn't say that it 'makes it ok', but I do think it changes things if the child has been vaccinated. In that case the parent of that child has done everything within their power to prevent the child from getting the illness and unknowingly passing it onto a more vulnerable member of the community when they are in the incubation phase and contagious.
For me, if my child was to get sick as a result of being exposed to an unvaccinated child who was unknowingly carrying one of these illnesses, I couldn't help but feel a bit bitter towards the parent who made the decision not to vaccinate. If the child had been vaccinated, then I think I would feel that the parent had done everything in their power to prevent it iykwim?
I once saw a Law and Order episode (great source I know, but did make some valid points) where a mother was charged with negligent manslaughter for not having her child vaccinated. In this case the child passed measles onto a baby at the park who then died as a result of the illness... I'm not saying that this is right, but then we do send people to jail for cultiple driving when they kill someone... Yes if it is because of speeding or drink driving it doesn't really compare, but when it's because they were distracted by something or tired, perhaps it does...
BTW I want to make the point that I think there are circumstances where it is inappropriate to vaccinate a child because of health issues etc. But where the child is healthy I can't help but feel this way.