Why exactly is thawing via hot water bad? I read all over the internet and labels that you shouldn't thaw with hot water? However, tons of people leave meat to get to room temperature, so how is thawing with hot water bad?

For instance, when I buy some Pork Loin and it's kinda frozen, I run it under hot water for 5 minutes, it defrosts, then I start to cook it. I'm unsure of the danger in that? Yes It may "start" to cook running it under hot water, but I'm about to cook it anyways.

Or is this a case where they mean thawing with hot water and letting it sit there for hours?

Edit: I am using a Slow cooker however, I'm going to put it on high. (The only reason I even thawed it is because it was frozen "stiff" and was too long for the crock pot, once I thawed it could bend in).

But it seems like a Crock Pot for instance would keep things at a "below" cooked temperature for a little bit anyways as well....(at least on low) before it started to cook things. So why don't crock pots make us sick?
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.