Scientology is not a religion, it is a dangerous cult.
It's utterly outrageous that we have a law which supposedly prevents individuals from expressing their opinion.
3. A system of intense religious veneration of a particularperson, idea, or object, especially one consideredspurious or irrational by traditional religious bodies;as, the Moonie cult.[PJC]
even if you don't like that, there's:
2. A system of religious belief and worship.[1913 Webster]
so that covers every religion as a cult. As for dangerous - they have a well documented history of legal threats, intimidating behaviour, etc.
Update: It seems that sanity is not lost and the guy isn't going to be prosecuted because, as should have been fairly obvious, his sign wasn't inciting anything \o/