I flat towed a 2014 Ford Taurus for several years, that had that same 65 mph restriction, and often wondered the same thing.
I never really got any answers on why Ford restricts it that way. I suspect, like you said, that it may be more about legal stuff than it is any physical limitation of the car to be flat towed faster than 65.
I did not make it a habit to go over 65 when towing the Taurus, usually set the cruise control at 65 when on highways. There were many occasions, though, that it did get over 65 for a minute or two when passing slower traffic, or when going through the mountains. It did fine, and car is almost 10 years old now, is my daily driver. It has over 70k miles (and probably 20k more in towing miles), and still drives fine, like new.
I'm not advocating that you can totally ignore that 65mph limit, as there may be a reason for it other than legal. Just saying, it probably won't be the end of the world if you occasionally do go over such when you need to, briefly.
Anyway, we now tow a Jeep Wrangler, that is much easier and more simple to tow, and has no such speed restrictions. The Taurus is still set up for flat towing and could still be towed, but we don't tow it now that we have the Jeep, as the Jeep is a much more fun vehicle to bring along. 🙂
Will and Cheryl 2021 Newmar Baystar 3014 on F53 (7.3 V8) Chassis ("Brook") 2018 Jeep Wrangler JK ("Wilbur")
And then there is the press. EV fires get a lot of news because the fires are difficult to put out and the resulting picture is newsworthy. But do they every report on the numbers? Everything I've read indicates the per 100,000 vehcles ICEs have much more frequent fires.
2009 Holiday Rambler 42' Scepter with ISL 400 Cummins 750 Watts Solar Morningstar MPPT 60 Controller 2014 Grand Cherokee Overland
I usually cruise at 55 mph with double benefits: my navigator enjoys the ride more and I get almost 10 mpg.
However, but I loathe to waste the momentum gained downhill so let the speed rise to 75 mph guessing (without any training or knowledge except what you've shared) that the stress on the drive train will be minimal.
2000 Country Coach Intrigue 36' 350 Cummins ISC 2015 Lincoln MKX FMCA 351235 Look for us at Civil War Reenactments and the SCA.
The faster the speed, the more load on the drive train components. With the engine not running, the transmission is lubricated by the residual oil setting in various places. The engineers calculated how long this oil would last to provide for the safety of the transmission and it's longevity. You can tow at 80 MPH if you don't intend to keep the vehicle very long and will pass the problem onto the next owner. Why the rush, I set back at 58 MPH when the Allison shifts into sixth and enjoy the drive.
U. S. Navy (RETIRED) 1993 1995 Gulf Stream Scenic Cruiser 34' Diesel Pusher Cummins B5.9 12 Valve Engine, Allison MD-3060 6 Speed Transmission. TOAD: 2019 Jeep Cherokee Trailhawk Elite Road Master Sterling All-Terrain, Airforce One