The rats are not being placed in boiling water, they are placed in warm water. You don't want the water to be hot enough to cook the rat while it is thawing.

You can also leave them out at room temperature to thaw for a few hours like you would with chicken, or something you were gonna cook for dinner, and then when they are thawed you can place them in warm water for a few minutes to heat them to a temperature that will get the snake interested. If they rodents are already thawed, it only takes a few minutes (10-15) to get them nice and warm for your snakes. That's how long I leave mine soaking after they have been pre-thawed, and I have no problems.

The most important thing is to make sure that the rodents are completely thawed.