Is "English only" really unhelpful for Taiwan's learners?

A mass of evidence supports that using learners' first language is helpful for their learning of a second language. That's why a bilingual local teacher is much superior than a monolingual foreign one. Education experts in Hong Kong are now advising teachers to use Cantonese effectively in English classes. In the past, only English was to be used. A bit of common sense is enough to understand why it is much more efficient to use students' first language when teaching some abstract concepts or tongue positions in pronunciation, etc. Why, then, is it that monolingual foreign teachers are still so valued in Taiwan?

Aside from strong parental support for white teachers, hence the need for white faces on prospectuses, I think "supposedly" monolingual teachers are valued precisely because they are considered "English-only" teachers. I say "supposedly" because I know most of the foreign teachers are multilingual and can speak Mandarin very well, but Taiwanese people "presuppose" that only English is to be used upon seeing a white face. Learners will feel obliged to speak English, which they won't in the case of a local teacher. They feel awkward to be speaking in English to a teacher whom they know they can communicate much more efficiently with in Mandarin. Thus, they accomplish the goal of practicing spoken English better with "monolingual" white teachers than with bilingual local teachers.

On the other hand, some Taiwanese people seem to presuppose that people with East Asian faces necessarily speak Mandarin, and therefore it is awkward to be speaking English to them. I'm Japanese and I'm in a classroom where the medium of instruction is supposed to be English. So it should be natural that they should be talking to me in English. But when I try to talk to them in English, they tend to insist on using Mandarin in response. It's sometimes REALLY hard to get them to say any English word, even just "yes". They would insist on saying "對" instead! What a contrast with a Caucasian colleague of mine, who speaks better Mandarin than I do! I often overhear students doing their very best to answer him in English, even though the question was just asked in Mandarin?

Is it just me? Is it because I don't look at all like a non-Mandarin-speaker? Would they try to speak to me in English if I looked more stereotypical Japanese, with an anime hairdo and really tight clothing? (Thank you, Lauren, for your advice!) What about ethnically Chinese teachers who grew up overseas? Since their surnames sound Chinese, wouldn't people expect them to speak Mandarin all the more?