I don't know why they "should" match the "no tax" with Crutchfield, which isn't really a promotion anyway. The customer is still obligated to pay the taxes, from a legal standpoint. If they choose to ignore the law, which obviously everyone does, that shouldn't be Best Buy's problem. After all, Best Buy still has to pay the taxes, even if they are "matching" Crutchfield.
You're probably right, I'm just going off "casual" conversation with a sales rep when I was trying to negotiate the cost down. He seemed to think that it wouldn't be a problem, but I wanted lower and the talks went nowhere else. I would think if you walked in with a Crutchfield quote, delivered, and there was no tax, and said you'd rather buy from Best Buy and pick it up, they'd accomodate. But, I could be wrong.