Regardless what they think they are doing, they are still stating their own opinion. If we can try to interpret things in this way, even "I am stating a fact" becomes "I believe I am stating a fact."
Setting the term "facts" aside for a moment, I don't perceive a difference between "statements about the world that we believe to be true" and "opinions."
When people make statements about the world that they believe to be true, all of their subsequent actions reflect that high priority belief, and disproof causes major re-evaluation of everything.
When people express opinions, they may not act subsequently in ways that are in accordance with the opinion, and may change the opinion with more or less disruption to the rest of their actions.
I used to like cottage cheese, and don't anymore. This has no major effect on anything except my consumption of cottage cheese. I believe in everyday physics so much that I consider them facts, and would have to restructure everything in my life should gravity re-adjust to being 5% stronger tomorrow.
The burden of proof is different.