I can't comment on the other brands, I might recall that Sony does the same. Best tp check the websites and T&C for warranty.
Since the warranty is tight to the equipment (and not the owner), Canon simply goes along the sales date as only key factor for warranty. Otherwise it might end up in a lot of discussions about Tom, Dick and Harry having different opinions.
Secondly, I have not heard about Canon verifying identity by IC or passport when bringing in items. So if they have a different owner in their system they simply play along as long as the item is under warranty. It's a grey zone, it works.
The topic of warranty cards is very old (check the posting date) and it has become meaningless by now. There was a time when people had to send in this card filled with their details in order to start the warranty period. Some smart people tried to squeeze out a few more months of warranty by sending in the card later. With that idea, they offered hardly used sets with unfilled warranty cards into second hard market with the promise that if the buyer sends in the card the warranty would only start then. This way they tried to increase the selling price.
That all got meaningless when Canon used the date of sale as criteria and had all sales tracked in their systems.