Google is making the (accurate) calculation that over 99% of people coming from a Japanese IP address whose computer says "Accept-Language: en-gb;q=0.9, en;q=0.7" are actually Japanese speakers of Japanese (true preference: "Accept-Language: ja-jp;q=1.0; en;q=0.000001") whose configurations have been borked at the manufacturer or the IT department level, and who will be terribly served by content written in English. It's an act of charity that they even ask about English prior to defaulting you into Japanese. (They haven't always done that -- every year or two it seems to switch for me.)
This is a particularly acute issue since most users of Google don't actually have accounts on it, their best attempts to fix that to the contrary. As a result, in key ways such as language, the service needs to "just work" for the majority of their users. Also, if I was to have a totally clinical view of the situation, the expected value of 1 English speaker in terms of ad clicks is ridiculously below the expected value of 1 Japanese speaker in terms of ad clicks, given that both are in Japan (+), and you're asking to sacrifice 99 Japanese speakers to save the 1 English speaker. Once you accept that there will be a loser in this decision either way, figuring out who the loser should be is not that hard. Sorry: I'd prefer English, too.
>> And by the way, want to check out a post on Blogspot? Off to blogspot.jp you go. This doesn’t change the page itself, only the URL. So what does redirecting me accomplish, except make me feel that I’m not in control of what sites I can access? >>
This is a hack around censorship laws in a few places (most relevantly to the instant case, Germany, I believe). Google has a blogspot.jp which is a copy of .com which is a copy of .de which is a copy of... except that they enforce country-specific censorship laws. This was deemed an acceptable tradeoff versus getting all of blogspot blacklisted in e.g. Germany.
n.b. No special inside knowledge here.
(+) If this is not intuitively obvious to you talk to me about AdWords geotargetting and default settings sometime.