Because everyone can discern the effects of a complex piece of legislation just from reading it.
You are too simplistic. The Navigation act of 1817 was one component of the problem. Federal subsidies for northern shipping such as mail carrying contracts, fishing vessel subsidies, and tax money to finance improvements in rail lines, canals and harbors is also a factor.
The warehousing act also helped. There were several acts passed between 1817 and 1860 that benefited the northern industries at the expense of the southern industries.
But what would have happened immediately with secession is that the prohibition on using foreign ships and/or crews would have disappeared, and the European ships would have taken over the entirety of Southern import/export trade, and this would have cut out the northeastern shipping as well as 72% of the FedGov tax base.
The products manufactured in the north would lose their southern markets because they would be displaced by lower cost and better quality European manufactured goods.
It was many hundreds of millions of dollars per year which would have been instantly lost to the Northern industrialists and power brokers.
So they got an army to invade to stop the money loss. And then they pretended they were invading for some other reason.
“… tax money to finance improvements in rail lines, canals and harbors is also a factor.”
The fact that Southern ports were dredged using federal funds has already been pointed out to you.