I'm a retired truck driver with lots of flat bed and some low boy experience. The rule is pretty much that the load needs to stay put in the trailer under all driving conditions. That's my wording, not the law's.
That driver in your photo would not need to use any additional securement if the skid steer would fight tightly in the trailer on all sides as long as the trailer is sturdy enough to handle the driving conditions and, of course, the truck and trailer are legal in all other respects. You do not need to strap, chain, etc. your load if it will not shift. It is not required that the load be secured enough to stay on the hauling vehicle if it rolls over into a ditch, etc. but it better stay in place if he has a 'typical' fender bender. Load shifting is prohibited.
The pictured trailer and load has several obvious problems. The sides are marginal appearing as far as sturdiness, marginal as to whether they are high enough to keep the skid steer from tipping out on a hard turn, and obviously the machine doesn't fit tightly in the trailer so it does need to be chained or strapped properly. Clearly no knots except for tarps and other load coverings as long as the coverings do not shift.