If that is indeed the case, than the government which would adhere to such a definition is itself calling upon a doctrine no different than the old monarchies of Europe - that the might of the state determines right from wrong, and that any resistance to that might, no matter how externally legitimate, qualifies as "treason" on the grounds that it is a form of resistence.
Spooner's argument counters this possibility by presuming, as had universally been admitted, that the United States came into being on greatly different grounds than the old European monarchies where the king was the state, the state was right, and any resistance to the state or king was therefore in the wrong. If indeed the United States formed under different terms, then the act of secession could not have been treasonous. If the act was treasonous though, then the United States would be exerting claims to a power of no noticeable difference from that claimed by the European monarchies, thus meaning that the entire theoretical premise they claimed for exerting that power - to preserve the American "experiment in self government" and prove to the world that it could succeed - was nothing more than a sham.