I read through the comment threads multiple times, and there are some that make a point... And I'm sorry to pick out individual posts that I disagree with, but there are some that are, in my opinion, a bad precedent.
This one for example:
The more I think about this, the more I think it is not a good
question for this site. "How do you justify...?" is accusatory in
tone. It implicitly assumes that everyone is morally bound to accept
any other person's position, which is an invalid and unsubstantiated
assumption. As noted by others, JW and LDS each are guilty of the same
thing. So, no position can ever be considered invalid or heretical?
This whole point of the question sounding accusatory could have been handled by just editing the title to say "What is the basis for..." instead of "How do they justify..." If you read the body question itself, it doesn't sound accusatory, it sounds bewildered. The "What is the basis for" is certainly allowable.
With that edit, the next comment also becomes moot:
This is a bad question for this site. Specifically the OP's assertion
that mainstream Christianity is united and speaks with one voice.
Those of us that have been on the site already know that "mainstream Christianity" doesn't speak as one voice on a lot of things. Heck, it's the basis for my meta post [Is it valid to ask if “Christianity” teaches anything?]. SO I somewhat understand where this post is coming from, but you'd be hard pressed to find any group - Catholic, protestant, Orthodox, etc. that doesn't cite the same objection to each of the two groups listed.
Also, DJ Clayworth's excellent answer on that question addresses the basis without partiality of condemnation, much better than my own answer on that question. The correctness of his answer is self-evident to LDS, JW, and pretty much any "mainstream" Christian out there. So this question passes the "[real questions have answers not opinions]" test for StackExchange sites as well.
Finally... And this is the one where I think the precedent is dangerous. The first sentence in TheFreeemason's comment:
I believe this question is a question asking to define what it is to
be Christian (proof by contraposition) which is against the FAQ.
I don't see that anywhere in the question at all. What I see is someone asking what the basis is for something that's controversial. There's nothing in the question that is asking whether or not the basis is true, it's just asking what the basis is.
In my opinion, we cannot allow ourselves to close questions just because we think someone will get offended. There's a difference between asking if Jehovah's Witnesses are Christians or not, and asking what the basis is for not accepting them as Christians. it's not just semantics, there is a qualitative, difference between the two.
One is asking for opinion, and one is asking what's the basis for the opinion.
The difference between the two is that the one asking for the basis of the opinion is already implicitly acknowledging that there is disagreement, that this is just an opinion, and not a fact, and it's asking "Why do people with this opinion think that way?"
The reason the precedent is dangerous is that if we're ruling out questions about what the basis is for a controversial belief, what else is there to ask h everyone is Calvinistic in their beliefs... Do we outlaw questions like "What's the basis for Calvinism?" Where does it end?