a helicopter is a delicate machine, and perhaps just the fact that it was hit caused it to go out of control...There has to be some explanation, they wouldn't do something so blatantly obvious if there wasn't an explanation, and i'm not a mechanic, so i don't know, but perhaps the bullets hit something else, besides the gas tank, like the gyroscope, etc, so that it couldn't function very well anymore...
Maybe the explosion is a mix between the propellors hitting electrical wiring in the building (which any building has, i think) and im sure there is gas left in it...but helicopters are strange and if it senses something wrong it will go down. But im gonna stick with the electrical wiring in the building...plus there is surely oil and other flammable liquids in a helicopter. but what do i know.
Oh course, if you know engines, then decompression of the gas tank would make it not being able to deliver them as fast as the engine requires. Unlike cars, the helicopter's engine has very precise oil requirement amd injection rates and if there was no pressure then the motor would start to studder and eventually cease, full tank or otherwise.
Also, for the nit-pickers, the fuel gauge is also linked to pressure. Apparence if a lost of fuel can (possibly) happend with a pierced and decompressed tank.
Just my 2 cents, and right as far as I know... but I don't know ANYTHING...