No "grey area" at all in Illinois:
Illinopis General Assembly
Public Act 093-0344
"(b) Performance ability of brakes.
1. The service brakes upon any motor vehicle or
combination of vehicles operating on a level surface
shall be adequate to stop such vehicle or vehicles when
traveling 20 miles per hour within a distance of 30 feet
when upon dry asphalt or concrete pavement surface free
from loose material."
Nothing "grey" about that, at all, that I can see. In fact, it is the most restrictive law that my research has ever uncovered!
Yes, I towed a 2006 Jeep Wrangler Rubicon Unlimited, a 2001 Dodge Ram 1500, and a PT Cruiser convertible on a dolly on various trips around the country with no auxiliary braking system. Many trips crossed the Continental Divide, the Cascades, and the Sierras. No problems.
I sold my DEMCO Kar Kaddy dolly because it had no brakes. I have since realized that it was an incredibly stupid thing to do!