No, it's a consequence of normies.
The original Reddit, like Slashdot, and Dig to a lesser extent, defaulted to a global community. These places started out by attracting nerds from across the globe. Nerdy communities are internationalist, and fluent in English. So there was a tacit understanding that English was simply Esperanto and that anyone could be from anywhere.
Slowly getting mainstream, Reddit started attracting randos. And American randos apparently associate English with themselves, completely lost to the fact that something like half of the planet uses it as a common language. So I sat and watched as the overall tone of the place went from a spaceship, to Chick-fil-A, Podunk.
Sauce: English as a second language and been there from the start.
I don't think you understand. How do you know that the heavy majority of the users (of the early Reddit) were from the US? You are repeating the viewpoint that the place started out as US-centric — any concrete data here? Why do you think that the country where a web site was built in matters here?
I'll reiterate: English is fundamentally different from Japanese because it is widely used as a second language.