For the convenience of having it right then and there as opposed to waiting the day or two it would take to be delivered is the only reason I can think of. I know there have been a few things I have purchased from Amazon recently that if I could have taken the 10 minute drive to Best Buy and have it right then and there, especially on a weekend, I would have.
Pre-tax match won't convince me to buy from your stores. Try matching the EXACT amount coming out of my wallet and you might have a convincing policy.
Why would anyone want to pay tax and deal with a store with cr@p customer service?
That may very well just be me though.