This is direct proof that someone is using Windower, though. In general it's difficult to differentiate someone using windower and someone not using windower, unless they dug through the files installed. With direct proof comes punishment. Technically, this is a "game advantage" as someone who can speak Japanese and type in it now has advantage over people who can speak Japanese but cannot type in it (PC legit/PS2/XBox360).
I've personally argued for either being able to purchase the usage of the IME as a foreigner or allowing us to copy/paste into legit windowed mode myself. Anything to allow us to speak to our Japanese friends more successfully is welcome in my book. The biggest reason they ignore us is because they don't want to deal with English and Romaji, and I can't say that I blame them (though it does piss me off when auto-translate goes ignored, but I digress)...
Basically, Windower is against the ToS. If you make it blatantly obvious that you use it, I'm fairly sure they'll do something about it. Using Japanese on a client that is not Japanese is a good way to shout out your console of choice. From what I've witnessed, it seems in terms of Windower itself, it's a "don't tell don't ban" kind of policy.
Odds are high this topic will be locked soon for guideline boundary material. At least you'll know there are people supporting Japanese IME for nonJapanese players.
