Linguistically, it's actually an important qualifier, not a knock on an ethnicity that it was never intended to be. To treat it as non-relevant is to ignore science. Structurally and grammatically English is almost the complete opposite of Chinese, Japanese, Korean or many other Asian languages. I know from actually studying both Spanish and Mandarin.
Thoughts from any non-English native language, when transcribed into English, do not come out as linguistically correct as they would for a native English speaker of an equivalent skill level. This is true for any person and a conversion from any language to any other language.
Those whose native language is European-derived (Spanish, French, Portuguese, Italian, etc.) are going to more easily produce a transcription into English that linguistically makes more sense, since linguistically the languages have more similarity and the writer is going to have more luck in phrasing a sentence that makes sense in English. Someone whose native language is Asian-derived is going to have a tougher time and have less luck, not through any fault of their own or lack of skills/ability -- it just takes 3-4 times the amount of effort due to linguistics.*
To not mention that and just generically say "non-native English speakers" understates the skew to which the English writing is likely less than perfect due to the inherent linguistic challenges present in the population, all other things being equal. That's all.
*http://www.effectivelanguagelearning.com/language-guide/language-difficulty
