With respect Sober Lark, I know these Indonesian airports pretty well and, as you say, they have to live with the constant rumblings of Merapi and all the other active volcano's in the vicinity.
But, when Merapi erupted last year, it was not on the scale of Iceland and although there was disruption, most of the ash was blown away from the area. In addition, you are talking about some quite small airports in an area of low density overflights.
Finally, the attitude to risk within Indonesian aviation, although vastly improved on the past, is not quite the same as that in Western Europe.
As a western airline working out there for the Hajj season, we took pretty much the same view as people are here. IE don't fly in the red zone on the met chart and know your Volcanic Ash Encounter SOP's well!