I think I'd rather you bend your frame when towing inappropriate loads than the hitch intentionally shear off and your load goes independent on the highway.
We aren't talking "towing of inappropriate loads" levels here, and trailers heavy enough to cause such worries (by being too heavy for the frame of a, say, Prius, not just overloading the transmission/brakes) really ought to have their own brakes.
The only reason these forces don't already disintegrate the frame is that it's designed to absorb energy by deforming, so that if your stopped with you head nicely braced and a same model car (not truck!) rear-ends you perfectly straight with say 80-ish mph, you can have a chance of surviving, by way of your rear frame delaying the kick to your seat and sacrificially tanking a good part of the kinetic energy the other car brought to your situation.
Similarly, your front is designed to absorb energy to hopefully not rip your head off your neck if you run straight into a solid concrete wall, and do so while directing the crumpling to hopefully also not squish your legs (as you very quickly bleed out from the major blood vessels supplying your legs).
The only reason these forces don't already disintegrate the frame is that it's designed to absorb energy by deforming, so that if your stopped with you head nicely braced and a same model car (not truck!) rear-ends you perfectly straight with say 80-ish mph, you can have a chance of surviving, by way of your rear frame delaying the kick to your seat and sacrificially tanking a good part of the kinetic energy the other car brought to your situation.
Similarly, your front is designed to absorb energy to hopefully not rip your head off your neck if you run straight into a solid concrete wall, and do so while directing the crumpling to hopefully also not squish your legs (as you very quickly bleed out from the major blood vessels supplying your legs).