Do you think there's "something to-consider," when multiple manufacturers "just-say, no,'" or is it a tire industry-wide plot to get your money? Let's don our colander thinking-caps and think on this.
If you want-to use a car tire, it's your choice, just-like it's your choice to play Russian roulette, eat fatty foods, smoke (whatever), consume alcohol to excess, especially-before driving or riding; hang-glide, BASE jump, wear a squirrel suit and jump-off a precipice, engage in unprotected sex, or any-other activity which has the potential to harm you, and possibly, others.
Basing your decision to use a part not designed for the use to-which you're subjecting it, after viewing a youtube video is just a bad life's decision. Why would someone want to ignore multiple industries' engineering staffs (that would be at a minimum, the motorcycling industry and the motorcycle and automobile tire industries) because someone with a Go-Pro said, "lookit me! I didn't die! I did-it!" is truly a Darwin Award event.
One of the tactics to overcome the sidewall issues of car tire use on a motorcycle is to run the tire grossly-underinflated, so the designed tire pressure won't allow the sidewalls to function in the fashion to-which they were designed for use on a car. Excess sidewall flexing causes excess heat, and the tire bead is not going to be kept in-place as-well, nonwithstanding the fact the car tire bead isn't designed for the types of radial loads imposed upon it by being mounted to a motorcycle rim. Movement of the tire carcass against the parameters for which it was designed will subject the highly-engineered product to kinetic forces not planned-for in its design and manufacture, but I guess if some homeboy shot a Go-Pro of himself doing it, you bet your life! Because you are doing just-that.
You want to be a "darkside" rider, you should disclose to anyone to-whom you offer a ride pillon that your choice of tire isn't approved-of by either the motorcycle manufacturer nor the tire manufacturer, and let them be fully-informed, and to understand what you have chosen to-do. Think of it being in the same context as, "I just drank six beers, and now I want you to ride home with me on my motorcycle," or "my car has all-four tires showing cord, they're bald, and it just rained, but come accept a ride with me."
There is no reason to explain-away why a practice every motorcycle manufacturer would never admit-to on-record is a safe way to operate their product, is OK to-do; nor would you ever find a car tire manufacturer willing to say, "hey, it's OK to put that car tire onto a motorcycle!"
If you choose-to be that "darkside rider," and someone gets-hurt riding with you, the equipment you chose may be a contributing factor. Why take that chance to save a few dollars? That is a poor value judgement.
