Yeah, it's a fuzzy issue. There were the occasional person (like Taylor, and the first voluntary controller from Visser) who knew what they were getting into, and went voluntary anyway, in exchange for whatever they thought they got out of it. Those people were really traitors, and should be treated as such, but how do you tell the difference? With so many people who didn't know what they were getting into, even the people who really did act as true traitors could just plead ignorance, and nobody would be the wiser, so how could anybody be penalized for it?
And then there are the people who may have started out involuntary, but then eventually just gave up and stopped fighting it, because they figured 'what's the point?' Should those people be punished too?
I dunno, though. In the end, what difference did any of those people actually make? An involuntary controller and a voluntary controller both contribute equally to the invasion. For all practical purposes, there's no difference. Versus traitors in real-world situations, actually actively help the enemy, and contribute to their efforts. But once you're a controller, you can't possibly help the Yeerks any more than you already are, so what's even the point of prosecuting the voluntaries?