I doubt it has anything to do with the Japanese market. If you look at the bigger InsideOut releases in recent years, almost all of them had bonus tracks or bonus discs. Officially those are on the "limited edition", so it's a way to get people to buy an album sooner and pay more (when the album gets discounted, that'll usually be the standard edition with the bonus stuff missing).
This is true, but it's always different bonus tracks for Japanese editions. Even if some deluxe European edition has a bonus track, the Japanese version of the same album would have a different bonus track, not available anywhere else.
In some cases, the band would openly tell you to download or listen to the bonus tracks elsewhere, if you bought the actual album. That was the case with Sonata Arctica when they released either The Days of Grays or Stones Grow Her Name (honestly don't remember which one). European edition had one bonus track, Japanese edition had another, and I think the digital release (or some other release) had a third bonus track, and there was no way to get them unless you bought all three editions. The band's singer just said, well, everyone who bought the album is free to find the bonus tracks via any means available.