In this case the "throw" is for "shooting", and the phrase refers to the fact of "waste, waste goods that are not their own". There are several anecdotes as the origin of this locution but most mention the fact of Spanish Tercios soldiers saving ammunition and gunpowder in a siege until they capture the enemy powder keg, and already with the supply covered by "the king's gunpowder" (which they did not pay) the arquebusiers did not skimp on the amount of shots.
