First, the United States is a really poor analogue for Canada, because it has no federal sales tax. The principal issue does not apply in their case. They also do not collect state and local taxes at the border and, indeed, did not used to collect them on interstate shipments.
The big issue for us, even bigger than the duty issue, is the tax advantage that would be created. That does not exist in the U.S. because they don't have an inbound federal sales tax.
I think it's also worth noting what are loosely called “Amazon taxes” in the U.S. A lot of state authorities have starting imposing taxes on interstate shipments because their local merchants were being ground down, in effect, by certain merchants shipping interstate without collecting any taxes.
In many ways, they have gone in the opposite direction that eBay and Amazon and others are advocating here.