Their interest rate used to be <0%. To a first approximation that is worthless, there is almost no reason to buy Japanese government bonds. One Wiki page talking about 2013 [0] suggests that 70% of the bonds are purchased by the government itself and the remainder is mainly Japanese institutions that I would expect are legally compelled to (otherwise, why bother?).

I doubt the official Japanese interest rates are directly connected to the markets in a way that makes sense using traditional thinking. Most rational actors wouldn't be involved in a market where there is lots of downside and no real compensation. So the people involved aren't likely free to make rational decisions and all bets are off about what they do.

[0] https://en.wikipedia.org/wiki/National_debt_of_Japan