I agree with you in the sense that you expect to be treated fairly. But having collected many a bike in the past two years it is difficult to get everything together when the bikes old.
I have a 1972 Yamaha U7E that i bought with from a guy, who obtained it as a trade from the original owner. I have the original ownership (1973) but the original owner is dead. So i have a couple of options available and one of them isn't getting a bill of sale from the dead owner.
I judge each bike differently. ONE i bought last year, was from a different juridiction so i registered it here as being "new". It is easier then transferring it and cheaper.
Also when i sell bikes i do it carefully, and judge the person i'm selling it too. I had a honda scrammbler i was going to sell, but the guy wasn't planning on registering it to his name so i said no (and he was going to let his 14 year old daughter trail ride it -imagine a 30 year old immaculate bike being used off road -urgh. so i kept it :)