From an article posted at

The new wrinkle in the sales tax issue has to do with the distributions centers Amazon has around the country. The WSJ says [subscription required] that there are eight states with Amazon warehouses or distribution centers, but that Amazon has avoided collecting sales tax in those states by operating the facilities as subsidiaries of the parent company. Sales tax laws have permitted this exception when a facility is part of a separate legal entity.

Experts say that might not meet this exception, however, and might get snagged if the taxing authorities determine that these subsidiaries are merely shells set up to avoid the tax laws. It has been pointed out that even when items are coming from the subsidiaries, the Amazon website still shows the seller as Amazon, which would seem to weaken the argument that these are truly separate entities.
I posted this since there was some discussion on the validity of subsidiaries in "avoiding" state tax collection as we were exploring the New York issue.