"sales tax in California, which can be as high as ten per cent"
VAT, which is the European equivalent of sales tax, is around 15% in most of Europe and can go up to 21% is some countries. So why is 10% implied to be high?
Calling it the "Amazon tax" is also misleading, since the tax is paid by the consumer, not by Amazon. Amazon is just passing this extra take on to the state government (which needs it desperately since they're, to use the technical word, skint).
It's unlikely that such a tax will significantly hamper Amazon since including sales tax just puts them on the same regulatory playing-field as a high-street retailer. Amazon still has the volume purchasing power to be able to offer prices that undercut the high street.