Perhaps they're bait. Which is to say:

1. You want to make sure everyone in a community is persuaded of some fact.

2. But you don't want to expend unneeded effort in producing evidence to convince them if nobody needs convincing.

3. So you assert your conclusion as a bald statement.

4a. If anyone denies it, then you know there's a point in expending effort.

4b. ...or, maybe you'll just wait and hope that someone else gets annoyed with the denial and expends that effort for you.

Either way, you've increased the probability that your point will be made. The second strategy (#4b) is likely pursued more often than the first (#4a), because given a saturated community of "vigilant believers", the likelihood of the denial being fought will be nearly the same with or without you--and the cost will be much lower without you. I imagine these people write these as "drive-by" comments, not looking at the results, but feeling smug knowing that they've sicced their in-group on any vocal proponents of their associated out-group.