Who is buying that debt is really the Central Bank of Japan, so, it's really a political decision.
Yes, the central bank have a target interest rate and operate to keep it inside their chosen parameters.
Lenders accept that decision because they have not other option.
From http://bilbo.economicoutlook.net/blog/?p=40937 :
"That is what the Japanese experience since the early 1990s tells us. And all the stories about special cases; cultural peculiarities, closed markets, etc that the mainstream economists wheel out when another one of their predictions about how Japan is about to sink into the sea as a result of its public debt levels, or that interest rates are about to go through the roof because of the on-going and substantial fiscal deficits; or that inflation is about to accelerate because of the massive monetary injections; and more, are just smokescreens to divert our attention from the poverty of their analytical framework. The Japanese 10-year bond trade is called the ‘widow maker’ because hedge funds who try to short it lose big [..]"
(Does the central bank usually set the interest rate? Why do lenders accept that decision?)