
The strapping in is for the safety of the other passengers as well as for the safety of the wheelchair user if they are riding in the wheelchair.
A power wheelchair weighs between 250 and 300 pounds - without anyone riding in it. Not something you want moving around in a sudden stop, sharp turn or accident. My DDs power wheelchair weights about 300 with the batteries, her footrests and seat cushions. Her manual wheelchair is a lightweight one and weighs nearly 45 pound with everything on it. Again, not something you want flying around a bus. I agree with Bill, that part of the strapping is to keep it from toppling over sideways.
Also, for those who get out of their wheelchairs/
ecvs, sometimes that can save the driver some time strapping it in, especially if it doesn't have a lot of secure parts of the chair that he/she can use to attach the tiedown straps. It still will be secured well, but the driver doesn't have to worry about the passenger and can sometimes strap it in ways that are faster and would not be possible if someone was in the chair.
My DD can't easily get out of her wheelchair and, besides, she has been used to riding in it for many years, so she always rides the buses in her wheelchair. It's kind of scary though for some people who are not used to it. Besides that, ECVs are not designed to be sat on during transport - and many don't have any good points to attach the tiedown straps.