If you listen to the oral arguments, this issue was discussed at length.

There are two reasons for this distinction:

1. That's what congress decided. They get to determine tariffs, not the president. If the president doesn't like the law congress passed, he doesn't get to just ignore it.

2. Congress is very jealous of the right to tax and spend. They do not want to hand over this power to the president. Tariffs are taxes. If the president can just impose whatever tariffs he wants, he can raise revenue without asking congress for permission. That would grant the president enormous power to go around congress. Banning imports from a country does not bring in revenue for the president, so it doesn't pose the same risk to congress' power.

Trump has been trying to create a situation in which he can both raise revenue (through tariffs) and spend it however he wants (e.g., through DOGE's arbitrary changes to government spending) without ever asking congress. If he succeeds, the balance of power will be completely destroyed. The president will rule alone.