The Houston and Perth (has its wreck ever been found?) are in the Sunda Strait and if they have been stripped I would dare say it was by divers looking for souvenirs and the like rather than pirates. I had a look and you are right there are pirates in the Malacca Strait but I would guess a fair few hundred miles north of where this plane has gone down.
A site called EagleSpeak. US has an account of fuel piracy wherein they seize small tankers and lighters and syphon off the cargo to other vessels. The included maps show hits on tankers and others mostly south and east of Singapore. Looks like the authorities have mostly crushed the attacks in the Malacca Straits in the past decade.
When I cogitate on it I reckon the thieves may feel to exposed out in the open sea and further from their bases.
Www.abc.net.au/news/2013-12-13/outrage-as-warship-grave-stripped
Story is about Perth being stripped by salvage barges. Looks like most of the superstructure and the gunhouses are gone.