Deciding upon who to trade with is a decision left to the legislature as per the Constitution.
Trade with a foreign government requires no intervention by the public because there is no Constitutional change necessary.
When the people are asked to vote on referendum that would essentially change their way of life, and in effect, change the Constitution, then the question should be put to the public to be scroutinized. The Legislature should not be allowed to change what is layed down in the Constitution without the direct approval of the citizens.
That's why the Constitution starts with 'WE THE PEOPLE OF THE UNITED STATES', and not 'WE THE GOVERNMENT OF THE UNITED STATES'.