Steve, I acknowledge your point as a valid one, as far as it's limited to the idea that "slippery slopes are dangerous." Yes, yes they are. Particularly for controversial cases. However, as sometimes said in legal debate, yes, there is a slippery slope, but the slope won't slip in this case.
Scientology is not borderline. Here, there are clear limits we can set. The beauty of a democratic legislative system and common law judicial system is that it allows for heroic battles and reshapings in the gray areas as public opinion pushes for reform from both sides of the issue, while maintaining clear areas of wrong in the periphery. Under the current body of common law precedent, Scientology is too much of an outlier to affect any future decisions. Even in the unfair court decisions that exonerated CoS of some of its crimes, the judges basically say "only in this case, we make the exception, and for no one else," barring all future cases from citing the decision as precedent.
I repeat, Scientology is not a borderline test case. It kills and blackmails. It extorts vast sums of money from followers and threatens the IRS with frivolous lawsuits, bullying the IRS into a particularly sweet tax-exemption deal that no other religion in the US gets , mainstream or otherwise. Its silencing of freedom of speech is blatantly outside the range of judicially acceptable precedent, even in this era of large-scale threats to freedom of speech in America. The only question that is possibly controversial is the question of whether Internet reposting of unaltered copyrighted material is protected under free speech if its posted for the purpose of newsworthiness or inducing commentary.
You can feel good about opposing Scientology with all your heart. The line won't slip because of it.