1. People yearn for identity labels. It's a core part of human existance.
2. "White" and "American" are problematic identity labels. People therefore often reach back toward European ancestry (real or supposed), for identity labels that are less controversial.
3. The average person isn't aware (or concerned) that "Viking" isn't strictly an ethnicity. Because it's nevertheless a commonly used identity label.
Not everything has to be an opportunity to spot Nazis hiding behind every tree, or showcase your pedant chops. People wear shamrock jewelry or put an Italian flag bumper sticker on their car because it's fun and feels good, simple as that. Only a small number of legit white supremacists, and a legion of absolutely insufferable Internet progressives, think about this all that deeply.
1. You can't just steal someone else's identity labels when you've polluted your own so much that you can't stand them any more.
2. It would be so much more useful if Americans who didn't like what their country is doing actually fought for it to stop doing that, rather than trying to co-opt other people's identities.
3. Making up an almost-completely-bullshit identity marker like the modern version of Vikings is even worse, because it's not only stealing someone else's identity, it's then erasing that identity with some made-up bullshit.
Maybe y'all should start thinking about this deeply.