Kangaroo products, including skin and meat, are banned in the state of California. However, there is no federal, US-wide prohibition on either.
Customs inspectors at airports are usually employees of U.S. Customs and Border Protection (CBP) or the U.S. Department of Agriculture (USDA), both of which are Federal agencies. (By contrast, California Border Protection Station staff checking travellers coming in by road are state employees.)
So the question: if you arrive in California, and a CBP/USDA employee spots a prohibited-in-California-only kangaroo product, will they take any action? Or is this literally not their problem?
Obligatory disclaimer: I'm asking out of curiosity, not advocating breaking the law, and regardless of the answer you risk fines of up to US$5,000 and possible jail time if a California state inspector does show up at the airport.