DOT regulations are not the only regulations you must meet to transport chemical products into the state of California when those products are for commercial sale. California’s regulations do not overrule the DOT, they are in addition to them. I’m not sure what part of this is hard to understand. It’s not exactly a secret that additional regulations exist. Perhaps you have seen products labels “this product is known to the state of California to be a carcinogen” or similar in products outside of California? Yes? This is because it is required by California, but not other states. It is but one of many, many requirements specific to California.