Amazon also operates under multiple subsidiary names to avoid needing to charge sales tax in many states. But more states are writing laws to close that "loophole". I posted details about this (with a link to a relevant section of the amazon article at WikiPedia) in another thread at MobileRead.
Many "Brick and Mortar" retailers want to get rid of the internet "tax-free" ride for online purchases, so they have less "unfair" competition with online-only stores. Some of them complain that many customers visit their store to see and handle their products, then go home and place a cheaper online order.
Tax free internet sales for us in the USA may not last much longer, I am afraid.