You're legally required to pay sales tax on everything. It's just easy to avoid on out-of-state purchases. My company pays sales tax on EVERYTHING, even on out of state purchases. The state is more inclined to audit a company. Nothing more painful than paying a few years worth of back sales taxes on millions of dollars worth of purchases.
Nobody likes paying the taxes, but it's not a new tax. It's always been the law. They've just figured out how to enforce it. It will take away some of the incentive to buy out of state instead of locally. We've all done it. It's cost + shipping + transfer fee vs. cost + sales tax when you're buying a gun. From a politician's perspective, he'll be helping the in-state businesses by removing that advantage as well as getting some uncollected revenue. At the same time, he's voting against something we all love, but still illegal.