Except MBV isn't doing business in CA. They are doing business in AZ. Sure, CA can make whatever laws it wants, but those laws do not apply outside their borders. The legal question here is when/where does ownership of the product change hands from the vendor to the customer. On a pre-paid order, it's pretty clear that happens at the time the product is paid for and shipped.States have control over doing business in their state, it is about like importing from other countries. Each state is like a mini country in a way, all belongs to USA, but they have their freedoms to make laws for their own state. It can be very loose laws or very stringent. We are just seeing the beginning of all this sadly.
I've had my own interaction with with Cali. Their board of equalization hounded me for the better part of a year demanding I turn over all my records of sales to Cali residents so they could collect use tax from them. I kept round-filing their letters. When they eventually called me, I told them: Show me something in PA or federal law that requires me to do this, otherwise bugger off. That was several years ago. I never heard back from them.