But this year, Amazon switched teams, joining Walmart on the pro-tax side — not out of some newfound concern for “marketplace fairness,” but because Amazon’s business model is changing in such a way that now Amazon stands to benefit from this tax.

In order to provide faster shipping, Amazon is building warehouses throughout the country. These warehouses constitute a “physical presence,” which requires them to collect sales taxes, in any event. So, if Amazon is going to have to collect sales taxes under the existing “physical presence” doctrine, it may as well try to expand online sales taxes to whack its smaller competitors who don’t have a 50-state network of giant warehouses.

The rest of the online retail industry remains on the anti-tax side. NetChoice, a coalition including other online retailers like eBay and Overstock.com, is fighting the “Marketplace Fairness” bill.

Boo!