Intent matters here, and anti-vaxxers spread this information because they believe they will help the kids. Scientists are often wrong. Probably less often than other people, and probably not in the case of vaccines, but it is dumb to make it illegal in general to say things scientists disagree with.

And yes, scientists say that kids die without vaccines, some people disagree, so you would actually punish them for disagreeing with scientists since otherwise you wouldn't be sure these people killed kids.