Whenever fluent foreigners are asked/explained something in English, people make it seem as if a great offense has been committed. But really, it's just people trying to be polite and convenient. Theoretically, from the perspective of a Japanese person, I'd imagine that since there are most likely many more foreign people who understand English than those who understand English and Japanese, if they weren't to respond immediately as like Japanese, then I'd do my best to speak English so the conversation can keep going smoothly.
I think because we as foreigners have invested so much time in learning such a unique and difficult language, once we reach the level of fluency, we want to display our hard work and achievements, and maybe we expect to always be spoken to in standard Japanese. And when consistently prodded questions to in English, I can very well imagine how this can hurt someone's pride they've built as well in learning the language. Because of this, sometimes I think fluent foreigners are more sensitive to situations when Japanese people do their best to communicate in English. We do our best to blend in with the Japanese people by learning the language/customs/culture etc., yet at the same time some doofus will come and ruin our happy day of blending-in by yelling （ヘイミスター外人さん！）。
Well guys, it comes down to the fact that people are simply trying to be polite to us. We went out of our way to learn Japanese for their sake, so why can't they go out of their way to speak English for our sake?
Hmm, when I was talked to like a 5 y/o I really couldn't do much but think "Well at least they're trying". I mean, gosh, now that I think about it, almost every department store I went to I was 5-year-olded (I'm making that a verb now). Granted my Japanese isn't at the level of complete fluency (JLPT lv2 Dec3 baby! Wish me luck!) , but frankly whenever my ペラペラ host brother explained something to me I already knew about in English, I just felt that was thoughtful of him and not insulting that he was telling me something that I already knew.
I think it's our jobs as foreigners in Japan to tolerate 5-year-olding or recieving translation help from our Jspouses (lol Jrock, Jpop, JR,and now Jspouses, what's next?), and even if you are fluent, just roll with it, because they're trying to be just as nice as we are (most of the time).
... it sure is better than not knowing what the person is talking about.

O ya, any Japanese opinions on this subject? I'd be interested in hearing them