The Illinois Supreme Court threw out a state law Friday that taxes certain Internet sales, saying the so-called "Amazon tax" violated federal rules against "discriminatory taxes" on digital transactions.

The 6-1 ruling represented the first time a court had invalidated an Internet sales tax law among 18 states that have them. It brought an immediate cry from traditional, store-based retailers for Congress to step into regulating taxes on web sales.

The court determined that Illinois' 2011 "Main Street Fairness Act" was superseded by the federal law, which prohibits imposing a tax on "electronic commerce" and obligates collection that's not required of transactions by other means, such as print or television.

Illinois' law required out-of-state retailers to collect state taxes on annual sales of more than $10,000 that involve in-state "affiliates," or website operators and bloggers, that draw consumers to the retailers' sites in exchange for a cut of each sale.

That prompted several high-profile departures from the Prairie State by companies such as CouponCabin.com, which fled rather than lose so-called "click-through-nexus" payments from the Internet retailers.

But Justice Anne Burke, writing for the court's majority, questioned whether there was any substantial difference between out-of-state businesses reaching Illinois consumers through a click-through-nexus approach or through other approaches that aren't taxed.