My understanding of the U.S. use of chains is to keep the trailer somewhat inline behind the towing vehicle in the event of a separation from correct connection between the trailer and the tow vehicle. I was indirectly involved with a 30' fishing boat/trailer equipped with a 2" hitch being incorrectly hooked to a 1-7/8" ball on the truck, resulting in a decoupling on the 405 freeway near LAX some decades ago. The tally was one Sigalert and a very beat up tailgate on the pickup truck, but no other vehicles involved, because the trailer stayed behind the truck. With two, 150 hp Yamaha outboards on the back of the boat, pitch-pole prevention was not a concern, and really isn't with any other trailer size/type. Have been out of the boat biz for a long time, but I don't recall any trailers from the major manufacturers who rigged trailers with chains so short as to keep the trailer tongue from touching the ground in the event of an uncommanded uncoupling. In an uncoupling, the chains are for safety to other vehicles, your tow vehicle is a sacrificial anode.

I don't know if it is written anywhere but just from my general up bringing towing horse trailers, utility trailers, campers, and boats, we always crossed the chains. In the event of trailer separating from hitch, the crossed chains allowed the trailer tongue to rest on the chains. The case of the utility trailer coming uncoupled from the ball was when I borrowed a utility trailer and it did not have a safety pin. I was only using in my neighborhood so the first time in all my towing I did not use a safety pin. The road was rough, the safety latched popped up and then the coupler popped off the ball. It probably is not needed to cross the chains and may have started when chains were too long. But the brake-away brake is sure to help with pitch polling. A disconnected trailing traveling down the road by itself could easily have the coupler dig into the side of the road and kinetic energy allow the trailer to pitch pole. And I can see now why chains are not required and brake-away brakes are for the UK, since they use smaller vehicles.

I've seen many accidents involving trailers along interstates and my speculation is probably due to loss of control by swaying. I have pulled trailers to their limit until they swayed and its no fun. At least I knew at what speed to stay well below until the swaying would start. Some trailers sway at a much slower speed than others due to heavy in the rear, not enough tongue weight, or improper placement of axle. I am very pleased with my 1975 Pfeiffer trailer and the way it pulls. It does pull even better now with a little more weight in the front when I added a spare tire and small tool box.