To a certain degree, your correct. It is always a personal choice to tether or harness a dog in any vehicle. Your even totally correct that they can indeed become loose objects in an emergency stop or even by their moving into the driver's seat or beneath the driver's legs while he/she is driving.
I have ridden with my dogs for many, many years in both my previous Model A's and in my sidecar rig. I have tethered them and also ridden without such. Each has a certain risk factor built into it in a manner. The 55-70 mph speeds we generally ride at in my sidecar rig actually reveals that no tether, harness or even super glue will likely do all that much in a major crash for them and can just as easily hold them in place to suffer injuries they may not otherwise. A dog harness is not quite the same as a seat belt in design or in function when you really look at them.
The one time I did flip my sidecar rig over on top of me, I was very glad the dogs were not tethered or harnessed in the hack. In fact, it was the one time I didn't even have them riding along with me when it happened. If they had been, it would have been highly likely that the tether would have kept them secured, but allowed them to drop right down onto that right side muffler or set of red hot jugs below and kept them there scalded until the rig could be righted. That 600 lb. rig was no light weight to have laying on top of a tethered or harnessed dog, too.
Again, it is totally a personal choice as to how one rides or chooses not to with dogs, cats or any animal inside a vehicle of any type. Like anything else in a wreck, they too can be loose flying objects and that is indeed factual. That said, tethers and harnesses do not have the safety features in design as a seat belt is for humans. Everything in life is a risk in some manner so make your choice and live with it. Me, I'll continue to ride with my dogs and enjoy such tethered or not. I have special seat belt adapters for installing a dog harness when I choose though. I have to install my seat belts in first though. All of my own dogs have always been well trained enough to stay seated where I put them as we travel. Since I do have a pickup with the wood stakes in back, I'll probably be adapting that area to make into a sort of dog catcher-like vehicle and probably let the dogs ride back there for the most part if all three are with me.
https://youtu.be/cw1grVC8Lkk
