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.
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Will and Cheryl
2021 Newmar Baystar 3014 on F53 (7.3 V8) Chassis ("Brook")
2018 Jeep Wrangler JK ("Wilbur")