The states, to date, have only required businesses doing business in the state to actually collect the tax. Retailers, not wanting to be burdened, have long since (and successfully) argued that if they have no employees nor physical locations in the state, they are not doing business there.
Frankly, states have long been after mail-order houses to collect and pay the tax, but without success. Now, in the era of the Internet, its a bit difficult to make the argument they are not doing business in a state just because they have no office. The COTUS action merely referees the issue in favor of the position states have long-held. Certainly the commerce clause allows the Feds to refer issues affecting commerce across state lines, which is all this is really about.