Frankly I don't care how many volunteers he has, dictator was about his ambitions, say what you will.
I imagine his ambitions stretch further than the current members of the party.
Rohm didn't force people into the SA, neither did Mussolini, Stalin, Sadam, Assad, Castro or anyone else. They surrounded themselves with people who wanted to take part.
BTW, if %99 are volunteers, what's the remaining percent?