Doesn't deciding that it's fair use specifically mean that they think it is copyrightable? The fair use doctrine specifically refers to the use of copyrighted material.
No, deciding it would be Fair Use even if it was copyrightable means you can cutoff the process considering copyright.
(Its perhaps useful to think of legal cases as consisting of a set of parallel questions connected by logic operators—once enough of them are resolved to reach a decision which no resolution on the others will change, the process is free to conclude without waiting for the others to be resolved.)
if (!isFairUse(workUnderInvestigation) && copyrightable(originalWork)) {
bigCopyrightPayout();
}