I've only studied two states so far, mine (Virginia) and California. Generally, the laws require equipment pretty much like what we'd expect for brakes, lights, mirrors, etc. I'm no expert on the law, I'm just reporting what it says in black and white, and in surprisingly simple and easy to read terms.
The California Motor Vehicle Code is found here:
http://www.dmv.ca.gov/pubs/vctop/vc/vctoc.htm
The Virginia Motor Vehicle code is found here:
http://leg1.state.va.us/cgi-bin/legp...cod+TOC4602000
Like Virginia, the concept of "overweight" appears to apply primarily to commercial vehicles in Califorinia. To be legally overweight, a vehicle would have to be more that 20,000 pounds on a single axle, or in California, 10,500 pounds on any single tire. I could find
no reference to the things we have been discussing, such as tow ratings, or any references to manufacturer's ratings at all, for that matter, with one important exception.
In California, it is against the law for a tire to be on the road above the maximum tire weight rating specified on the sidewall.
http://www.dmv.ca.gov/pubs/vctop/d15/vc35550.htm
For example, when a gross axle rating is the total of the two tire ratings, as it sometimes is, then the gross axle weight limit has the force of law in California.
The Point:
Manufacturers recommended towing ratings do not generally appear have any reference or support in the law. They are just guidelines published by manufacturers to define their recommendations for safe towing circumstances (and to control durability issues, as well). And it all comes down to one key safety concept: at the point of impact, there's always more speed happening than somebody intended.
In towing, there is no magic way to do it where you are absolutely safe, and the point where you are dangerous is a relative term which is always easier to find after the fact. There are no hard black and white boundaries that define good towing people and bad towing people. In between, there's a big gray area that we all have to operate in, and as different people, we have to choose different compromises as we work to make good choices to err on the side of safety.
I highly recommend following all manufacturer's towing guidelines. It is necessary that we follow the law, and the one universal and most important law for towing safety in every state is on the speed limit sign.
Note: If anybody finds something in the laws that contradict this, I'll cheerfully retract any offending parts of this post.