That one’s puzzled me, too. When my mother’s older brother died, she refused to attend his funeral or was it memorial service because he was being cremated and it somehow violated her beliefs. I think it has something to do with the belief in the resurrection in the Last Day. If the body is not intact, they believe it cannot rise to meet God, or something like that. I don’t understand that logic because we all know that dead bodies decompose, that some bodies disintegrate by the form death takes such as fire or explosion. But despite all this, I think that’s the reason. I think it is also the reason some religious people oppose autopsies; they think the body is no longer whole or intact for the hereafter.
In my mind, none of this fits the promise given to cripples or amputees and other severely disabled or ill people that in heaven there will be only whole bodied people, all illness will be healed. If God can restore amputated limbs, dead eyes and deaf ears, organs eaten away by cancer or removed by radical surgery—the list goes on—if he could create humans from dust in the first place, why can’t he restore full and healthy bodies from disintegrated remains and ashes?