I don't know why it's legal to carry any uncovered load in any vehicle.
Yes, it most certainly IS illegal to carry an uncovered or otherwise unsecured load in a vehicle (At least in the State, , , actually "Commonwealth", that I live in)
General Law - Part I, Title XIV, Chapter 85, Section 36
"Section 36:
Section 36. No person shall drive or move a motor vehicle on any way, as defined in section one of chapter ninety, nor shall the owner or bailee of any vehicle require or permit the same to be driven or moved on any such way, unless such vehicle is constructed or loaded so as to prevent any of its load from dropping, sifting, leaking or otherwise escaping therefrom, and, if it is loaded with sand, gravel, loam, dirt, stone, rubbish or debris that could fall on other vehicles or on the highway and create litter or potential hazards to other vehicles, unless its load is fully and adequately covered. This section shall not prohibit the dropping of sand for the purpose of securing traction, or the sprinkling of water or other substance on such a way in cleaning or maintaining the same. Whoever violates the provisions of this section shall be punished by a fine of not less than fifty nor more than two hundred dollars."
That's the State law side of it. Of course if someone's unsecured load is found to be at fault in an accident, the civil penalties can go as high as the cost of the damage(s) caused, which in many cases can run into many thousands, perhaps up to hundreds of thousands or even millions if the accident caused by the unsecured load results in death.....
"Civil penalties for causing death with an unsecured truck load vary widely, ranging from small traffic fines to over
$116,000 per violation for commercial carriers. State-level fines generally range up to
$5,000, while federal violations for gross negligence involving death can reach
$116,766. Families may also file wrongful death lawsuits for much higher, uncapped compensation.
U.S. Government Accountability Office (.gov)"
This is not "hearsay", I researched not only the State law, but also the civil litigation side of this issue before creating this post
Now, is it enforced? No, the cops ain't gonna be "cuffing & stuffing" the idiot whose empty Mc D's coffee cup blows outta the back of his pickup truck and hits your windshield. But if an 18 wheeler dumps a load of steel onto the highway and kills the proverbial "bus full of Nuns", well yeah that guy's likely gonna be doing hard time & the company he's driving for could possibly be bankrupted when all is said & done, especially if they've had prior incidents of unsecured loads causing mayhem on the road.