California residents who purchase from Amazon owe a use tax. Nevada Residents don't. The rationale is that if you are charging out of state residents sales tax when they don't pay it in their state, then they won't do business with you - unfairly creating an advantage for merchants in other states.

http://www.ftb.ca.gov/current/usetax.shtml

My company is pretty good about paying the Use Tax, and, in general, encourages our vendors to charge us California Sales tax where possible.