explain the legal basis... tl:dr Congress has for various historical reasons decided to cede more and more power over trade regulation to the executive branch

explain the legal basis... tl:dr Congress has for various historical reasons decided to cede more and more power over trade regulation to the executive branch over the years, starting all the way back in the 30s. This has resulted in the president having almost unilateral leeway to decide what constitutes "security concerns or unfair practices" when it comes to trade. And until congress passes a bill to reign in that power, it is probably legal for the president to decide that almost anything is 'unfair' and that basically any response is reasonable.