As Big Jim used to say:
Dry ice is frozen C02. It's temperature is -109 degrees F or -78 degrees C. Notice the minus sign. That's 141 deress COLDER than regular H20 ice. It's cold enough to burn you and cause you serious pain and injury. You may be savvy enough to use it with problems, but maybe some of your racing peers who see you doing it and try it are not. I don't want anyone to get injured.
Dry ice can crack the magnets.
Your skin can stick to it and have to be surgically removed and can cause serious burns at the very least if touched with the naked hand. Stay away from it. It gets the motor too cold anyway.
Additionally, it could be accidentally spilled on a neighboring person pitting next to you. Or if someone didn't know it was there, they may accidentally touch it.