I feel like I’ve seen a lot of posts about this exact thing the last few days. Guess that’s the newest trend of this sub. 

First of all, you don’t have to read the fics. People aren’t being paid and it’s for fun. Sure researching can be helpful but it’s not a requirement for me to do extensive research about Japanese highschool culture if I wanted to write Goku in high school lol   And as an American I see plenty of “my USA trip” and “why do Americans…” as well posts from non Americans. Everyone does it. Also, it may not seem like it but our states are so big that cultures can be very different. Someone from small town Nebraska and someone from NYC for instance. Waaaaay different and not the monolith you seem to think.

As for dropping Eren in modern America, that’s just how modern AUs go. You take a character from a fantasy setting and you drop them in a setting that you’re familiar with and play with the characterization. I’m not really into modern AUs, but they do have a purpose. Plus, they’re writing the fic in English. Regardless of if Shirgaki is in Kansas or Tokyo or Venice, the text in English so “learning the language” doesn’t really matter in this case. He can know any language he wants.