Because being against leftists convincing people to commit assault on nazis while also declaring anyone who doesn't agree with them for any reason to be a nazi is TOTALLY the same fucking thing as the isolationist mindset from before the Axis expanded their aim towards us and the world as a whole knew just how unspeakably vile the nazis actually were when they had actual fucking power, unlike the random idiots like Spencer.

Generally, it is better to avoid becoming entangled in a foreign conflict that does not strongly affect your own nation. However, Americans at the time greatly underestimated the ambition and reach of the axis powers. Powerful nations with a global endgame are not to be ignored. The preemptive strike on Pearl Harbor was a due wake-up call.