Earlier quoted context omitted.
So the internet imposes a moral requirement for all states/provinces to have uniform sales taxes or value-added taxes?
Not moral, but some would say being a single country imposes a certain practical requirement that the taxes not be too complicated, if we agree commerce is a good thing and waste is a bad thing.
The Commerce Clause gives Congress the power: "To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes." This has been widely construed as meaning that states cannot make taxes that affect other states' citizens unless they are doing business in that state.
The Supreme Court has just ruled that their earlier ruling (that Store A had to have some physical presence in State S in order for S to require A to charge sales tax) is wrong, and no such physical presence is required.