This always seemed like an odd conspiracy theory to me. It supposes that the U.S. is one the one hand so cavalier about dropping atom bombs that they're willing to destroy Japanese cities merely as a show of force to a potential rival, but on the other hand that they're so reserved about using it that they wouldn't use it to gain victory against the country they're engaged in a destructive war with.
For example, the fire bombing of Tokyo killed more in a single day than either Hiroshima or Nagasaki (though radiation sickness would eventually bring the Hiroshima total to slightly larger).
The only real difference with the atomic bombings was it was one bomb from one plane.
I would recommend reading into the firebombing campaing against the Japanese civilian population.
It was specifically aimed at maximizing civilian casualties, the same as burning of Dresden.
Something which would be interpeted as a war crime and terrorism today, wholly sanctioned by US military high command in the philippines at the time.
