I believe degree mills target the safety and health field because they see a group of people seeking recognition and praise, and feel they are easily duped. It must be true or they wouldn't keep advertising these degrees. Most people in safety and health have limited knowledge about how college degrees are granted. For example, name the body that accredited the school that you graduated from. Guess what, everyone assumes their school is legitimate.
Certifications seem like an easier target to criticize.
The answer may surprise you. There is a degree mill that started many certifications in the safety and health field, but that's another story. Maybe people in the safety and health field are not so knowledgeable on professional certifications, either. Professional associations could prohibit members from citing a degree from a mill as one of their credentials, but they probably feel it's not worth the legal effort, time and costs to do it. The National Society of Professional Engineers' Board of Ethical Review states that citing a Ph.D. or other degree from a degree mill is unethical.