Well there are three reasons.
The first reason I recommend being familiar with the OSHA regulations is that they are pretty much based on recognized standard safety procedures. Familiarity with OSHA regulations is pretty much familiarity with most, but not all, safety standards.
The second reason I recommend familiarity with OSHA regulations is that the exams are created by question submissions from currently certified safety professionals and industrial hygienists. The vast majority of these health and safety professionals are based in the United States. We spend much of our time ensuring our employers and clients are in compliance with OSHA regulations, and we are familiar with the regulations. We are likely to write questions based on our knowledge base which is strongly biased in favor of OSHA regulations.
The third reason I recommend using OSHA regulations and the OSHA website is that the information is free. They have great information at the website, and it is available at no cost. Check it out at www.OSHA.gov.