What about these export taxes that consumed much of todays debate? Why does our Constitution prohibit them in Article I Section 9?
According to Christopher Collier in his Decision in Philadelphia, export taxes were common among commercial nations in the 18th Century. It was actually more usual to tax exports than imports since they were paid by foreign consumers.
But the situation of America at the time was perhaps unique. The primary exports of our country before the industrial revolution were agricultural products from Southern States. An export tax on their tobacco, rice and indigo would reduce demand. Southern States would initially be in the minority in both the House and Senate. They could not count on moderation from their Northern brethren and thus pressed for an outright ban on export taxes.
Northern interests were generally in the carrying trade itself. In 1787 they still suffered for lack of business, because under the Articles of Confederation the US could not bargain favorable or even equitable trade agreements with their primary competitor, Great Britain. They were largely cut off from exporting to Great Britain, trade with the West Indies, and without a Navy were easy prey. What the North needed was a government that spoke for all the States and could negotiate for them, with powers sufficient to regulate and protect foreign trade.
Thus, import and export taxes were flip sides to the same coin. Both were part of the arsenal that delegates had to work out foreign commercial deals. To demand an export taxation ban was akin to fighting with one fist.
In a game of political chicken, Southern States held firm. On this and the protection of slavery, they would rest their decision to join the Union. Northern States blinked. Exports would not be taxed. In return, Congress was free to otherwise regulate commerce and enact Navigation Acts, slave importation was protected until 1808, and imported slaves could be taxed up to $10 each.