A league is constituted of several independent nations that have joined for mutual purposes (usually mutual defense), however the nations within the league generally still maintain authority over their own policy, international relations, and economies.
In a government, any states or provinces within it must follow it's directives, as it is the final authority and supreme law maker and enforcer.
Since the United States, as Jackson argued, was a government, it was illegal for South Carolina to violate a law of the Federal Government, and as such Jackson was free to force South Carolina to comply.
