It's not cruel, it's natural. Snakes have perfected their killing techniques for thousands of years. They don't suffocate or "torture" the rodent. They target the circulation system and stop the heart from beating. This prevents blood flow to the brain and any comprehension of what is happening.
Most people think that the constriction is associated with suffocation. That is actually not completely true because suffocation takes far too long to kill the rodent. If you time some of your snakes, you'll notice that their heart stops beating faster than it could from suffocation.
There is nothing cruel about nature's way of doing things. Cruelty is only done by humans, because we're the only ones that judge nature.