He cant order via a tweet. This is just an effort at friendly persuasion. But he can order a suspension of trade or a partial Sutor soon ( certain commodities, goods, etc) by regular presidential order - anytime he finds its necessary. So, by issuing this little tweet hes being very kind to big American corporate interests - giving them advance word that they need to get onboard with policy since important trade negotiations are in progress and stop undermining USA
Because government control of the economy is what conservatives have always stood for.