It comes from the fact that in most cases it takes at least 2 drivers to cause an accident, and likely one of them is at fault.Not sure where that assumption comes from. Though I would expect that a dash cam owner would only use the footage if it were to the advantage. I have one and most certainly would.
And your statement that as a dash cam owner you will definitely only provide your video footage showing who was at fault if it favors you supports my assumption. Thank you.