Because the rule of law is, that if you commit crime x you get punishment y. Lowering salary is not a regular punishment. If he would be fined with paying a fine the amount of his salary that would be ok with me. I'm not saying this man doesn't deserve punishment. But if his punishment was torture i'd also call that unfair.
Rule of Law means equal punishment for equal crimes regardless of the person committing them, so he should get the same punishment as someone who doesn't work at ICE.