It's not only California, almost every other state that imposes Sales tax also applies Use tax.
Sales Tax - Tax that is collected by the seller on behalf of the taxing authority(state/county/city) on the goods that are purchased within the jurisdiction of the taxing authority.
Use Tax - Tax which needs to be paid by the buyer to the taxing authority for the goods that are purchased from the seller who is based outside the jurisdiction of the taxing authority.
It is easy to enforce Sales tax because you can identify sellers within the jurisdiction based on licenses. It is difficult to enforce/audit Use Tax because it is voluntary. Most of the time the sellers are based in jurisdictions outside the taxing authority and so sellers cant be controlled by state goverment.New York Cigarette Tax
is one example of Use tax that was enforced aggresively.Wikipedia entry for use tax