Why blame "god" for natural disasters ? Its only nature after all
If you're asking why former believers or unbelievers tend to finger god for natural disasters, they don't. You can't blame someone you don't believe even exists.
However, an unbeliever can argue against believers who claim things like "god is all powerful, all knowing, and all good" by pointing out that if there is any form of human suffering -- particularly things that aren't the natural consequences of bad behavior -- god can't be all three of those things. He's either indifferent, impotent and/or unaware.
Believers, in turn, try to weasel out of this powerful logical conundrum by asserting that natural disasters represent god-ordained collective punishment for unusual levels of societal "sin", or that all suffering is somehow a result of sin or even just lack of sufficient piety, or even failure to belong to the right "in" group that will put you under some imagined "umbrella of protection". Or ... they can always resort to talk of god's Mysterious Ways. They also make bizarre, incoherent arguments about "free will" and how we would be "robots" if we weren't free to sin (and therefore, to suffer). Although somehow this is no problem whatsoever in a future heaven where people aren't fee to sin and where there will be no suffering as a consequence. If that can be accomplished in heaven, why not now?
It is the appropriation of the Causal Fallacy for the purposes of instilling fear and obedience in the ignorant and gullible.
Throw in a good dollop of the argument from ignorance and you have a useful theological weapon.