"required to prosecute all crimes to the fullest extent of the law", taken literally, requires prosecutors to prosecute everyone for every crime all the time. After all, you don't know what might turn up in discovery, anything could potentially have happened! Obviously, there has to be some judgement call made, where there's just not enough evidence to prosecute me for drunk driving even though I stopped an inch past the stop sign. Ultimately, that's just prosecutorial discretion again, and while it could be reformed and limited somewhat, it will always exist and be abused.