lack of regulations* allow, and no not exactly;

eu and canada have stringent laws on advertising to children, and laws on nutrition and additives to their products

even things like bread in the US have an insane amount of sugar

but the other reason is suburban/car culture & zoning, which means more hypermarkets and shopping not every few days for fresh food but every week or two for more processed food that lasts longer because going to the store is a bigger PITA than a quick 5 min walk to the neighborhood stores; which also means mroe walking rather than driving, another area of calorie burning and lean muscle maintenance which maintains high metab