I'm mean really...
Christian dislike atheist and athiest dislike christians. They will never agree.
Democrats dislike republicans and republicans dislike democrats. They will never agree.
We are all different in some ways. That doesn't mean there isnt a way to find a compromise.. except it seems not many want that. Its the old adage of "my way or the highway".
Humans are their own worst enemy.. And sadly the worst enemy to every other species.
Well, I've always heard that honesty is the best policy, so I agree that people should be honest about their beliefs and how they came by them.
I think a lot of it depends on the context of the situation, whether it's an informal or a formal setting. Message boards and social media are informal settings. They are not courtrooms, they're not classrooms, they're not scientific laboratories, they're not academic symposiums, they're not legislative chambers, and they're not churches or holy ground. A formal debate has a certain structure and rules. Moreover, form is often just as important as content.
Message boards, by design, tend to be a bit less formal and less structured. That's what some people seem to miss.
In an academic setting, academic honesty is what's important, not necessarily personal honesty. The idea being that people feel compelled to demonstrate they reached their conclusions/beliefs through rigorous scientific examination and study and that their own personal feelings or biases had nothing to do with their conclusion. It's a common trope where people say "this isn't personal, it's strictly business," and this is where some people may question one's honesty.
This can often lead to a popular sporting event one might see on social media or message boards, where people might goad and question someone about their beliefs, insisting that their motives are not objective or academically-oriented at all, that it must be due to some deep, dark personal motivation that one is refusing to admit. Or it might be due to ignorance, mental illness, stupidity, brainwashing, or any number of other possible causes.
To me, in informal settings, it's more like people having a conversation in a bar - or two old men jawing on a park bench. There's no need for the kind of stiff formality which might be more appropriate in an academic forum or a court of law.
I would wager that many personal beliefs are rooted more in value judgments where there may not be any definitive, scientifically correct or incorrect answer. Trouble is, most people don't seem willing to admit that, and that's where most public discussions tend to go downhill very quickly. I think this is what contributes to perceptions of dishonesty which you might be sensing.
It also seems to lead to misconceptions on the part of those who think that someone having the "wrong" beliefs must mean that they're not educated enough or they lack critical thinking skills or some other intellectual defect. Or it might be a moral defect, though that's more in the realm of value judgments where people can be a bit more cagey and vague.
If people have certain beliefs or ideals or values, there must be a reason for it - something that's a part of their inner being and internal psychology, which most people are reluctant to fully reveal in public or semi-public settings. So, they might put on a mask and act in a certain way that might seem like they're not telling the full story about themselves and why they believe what they believe. Thus, they might appear like they're not being completely honest and open.