In similar vein, mausoleums, raised tombstones, and cemetery shrines are also forbidden (the overall theme is "we are all equal in death").
The net result is that cemeteries are "reusable" and some of the most-popular in Saudi Arabia ("jannatul baqee'" a là Arlington Cemetery) are reused/recycled every so many decades (or is it once a century, once the denizens are forgotten/no longer visited?).
http://www.bury.gov.uk/CHttpHandler.ashx?id=4013&p=0
Specifically discussing Moslem burials, offers :
Types of graves available
Timber, brick or concrete vaults
On request, graves will be lined with timber, brick or concrete breeze blocks to form an inner shell in the grave space. Please note that we require the following notice for these types of vault.
And in NY (http://www.isnf.org/Documents/GuideToMuslimFuneralsInWestern...) you are charged extra for not having a lining on the grave.
"A concrete or other permanent outer burial container is not required, but there is an additional charge for a burial without an outer burial container."
The UK burial section for Muslims might be for offshoots or sects; for example, I do not know if Shiaa Muslims adhere to the same. Or it could be like here in the US: government regulations say you can't bury people sans-casket, so it could be "these (and only these) are your options."
Tangential: That's the commandment of Jewish burial law.
Bodies are to be buried in simple, plain linen or cotton hand-made shrouds. One reason is so that those who cannot afford fancy clothing aren't embarrassed that they don't have any.
Also, bodies are supposed to be buried without caskets, but in areas where local law mandates burial with a casket, that casket is to be simple and modest, rather than an ornate coffin.
I believe similar laws are applicable in Islam.
http://www.atlasobscura.com/articles/the-mushroom-death-suit...
Or what if the mushrooms holding your toxins are ingested by animals, would it pass through and then ultimately poison the earth?