Why exactly is thawing via hot water bad?
Thawing in hot water is unsafe for exactly the same reason that thawing on the counter is unsafe. You'd be very quickly raising the exterior temperature of the food to the danger zone (4-60° C), and allowing it to stay in that range for an extended period of time, in many cases more than the prescribed limit of 2 hours, and actually considerably less than 2 hours at "hot" temperatures.

Hot water or even warm water is arguably much worse than defrosting on the counter, because the exterior will heat up much faster than the interior. This might be OK for something like a 1/2" steak or a single chicken breast (even though it's still contra-indicated by every food agency) if the defrosting time is very short; but if you only have a small portion and want it defrosted quickly, you're far better off using the microwave which will do it just as quickly but much more safety and evenly. And for something like a whole chicken or a leg of lamb, it's almost guaranteed that the interior will still be nearly frozen by the time the exterior is above room temperature.

The reason for using cold water is to keep the entire item either below or at the very edge of the danger zone, where it can be held safely for several hours without much risk of bacterial contamination.