There's a nice take, at Nuerologica, with which I largely concur, on the whole Jesus cures cancer debacle in Napier that flared up a couple of days ago. It's a tricky line to police. In this case, given that the church wasn't telling people to discontinue conventional treatment and that they're not selling anything, fair enough, there's no real reason to force them to take the sign down. Though if either of those two conditions were violated, I'd be whole heartedly behind authorities taking a closer look at the issue.
The other, relevant point that is made is that while there is no real reason to force them to take it down, there's also no reason whatsoever not to mock it as the arrant nonsense that it is.