A law sets a limit of what is supposedly "safe", which in this case shouldn't vary from 1000 pounds in one state to 10,000 in another. For starters the capabilities of the TV should be factored in. So maybe 1000 pounds for a vehicle with a 3500 pound tow rating (minivan or small suv) would be ok w/o brakes, 2000 pounds would be ok for one with a 5000 pound rating (larger suv), and 3000 pounds max for any higher tow rating (full-sized pickup or suv). Wouldn't take much to make some calculations on reserve brake capacity in each type of vehicle and then set the numbers, then those numbers should be true in any state.
Like the inspection of amusement rides - which were handled at the federal level until congress stopped that in the early 80's. Now there is a patchwork of federal and state laws. A traveling carnival gets the federal treatment but permanent attractions may get full inspections from one state but none from the next state over. So it is safe to inspect a roller coaster here but also safe to not inspect a roller coaster over there? Inspections should be the default.
< gets off soap box >