It's about the publication in my opinion. It's not a race thing - it's entirely because of the publishing company and its standards.
Something ought to be called "manga" if it's originally published in Japanese, I think. That's just the most basic way to define it. After all, the right-to left format and general style of comic layouts comes from the Japanese language. No western comic is going to be published in japanese beforehand, so it can't replicate that style without it being forced. A second-language artist could publish in japanese in japan and call it manga, because it's written to the local comic book standards.
That's just coming from me. I don't really care what other people refer to as manga, even if it's factually iffy. You're free to think whatever, but I think it poses the possible issue of getting lost in translation. Hell, before you know it people might refer to all east-asian art styles as "manga" art. Oh, wait... my work gets profiled as "manga" for me being filipino so that already happens... even though the art style is a British one. I've literally had people say to me "your work reads confusingly" because they were reading it backwards on account of me being asian. It's ridiculous.
My point is, calling works "manga" that have no relation to Japan in their publication can cause these issues to a certain level... well, maybe not cause but more encourage. I follow a strict cut-off point, because it leads to less veiled racism in my direction...
Hell, as Kura already confirmed the same problems I have it's pretty safe to say asian creators are having issues because of this on a much wider scale, too.