They would probably never fall off in the first place, which is why you've never heard of anything remotely like this ever happening. Usually, there's two supports, one in the front and one in the back. Even if one of these large steel posts somehow fails, the other one will hold the logs long enough for the driver to stop the car. They've almost certainly tested stuff like this from a standstill while designing those beds. Same principle holds for ratchet straps, although there will be far more than 2 straps, making catastrophic failure even rarer, and those would be routinely inspected for damage.