lol. Actually you've got it pretty backwards. Until recently Japan was known as a country that rarely lip synchs. Its unfortunately starting to get more common, but its still better than most countries. I don't know about Korean acts these days (although it seems some artists are making more of an effort to sing live) but traditionally, Korea has been pretty much all lip synch all the time for the most part.
The reason why Arashi lip syncs? I personally think its because their live voices often do not mesh well with one another. Its obvious they weren't chosen as a group because they harmonize well together.
When they sing individually their voices are fine, but in total live performances when they sing together with no use of a back track sometimes it can honestly sound close to a train wreck. haha. Just check out old performances on Domoto Kyoudai for example.