Those are very good questions: the fact is that there is a huge amount of tension between the judicial and legislative branches, and within the judicial branch itself, about there the boundary between jduges' interpretation and reaasonable interpolation of teh law, and the text of statute as written. Conservative jurists like Justice Antonin Scalia think you should always go by the text of the law, and that it's dead wrong to consider legislative intent, no matter how obvious or well-documented it is/was; this approach (known as textualism) holds that if a law is no good, the correct remedy is for Congress to rewrite it. Judges should only dismiss a law as unconstitutional or go around it in cases where there is a clear and unambigious conflict between the Constitution and the statute. Other jurists, such as Justice Stephen Breyer, look at the Constitution as more of a framework document and think that you absolutely need to examine laws within the context in which they were passed and in the light of which problem they're intended to solve.

This is a very gnarly question, with good arguments on both sides - but in addition, there's a lot of unstated political baggage tied to both sides of the argument, so that what is on the surface a question of legal philosophy is on closer examination rooted in quite different philosophies of governance.

Now myself, I like the common-law approach and I would prefer a general class of crimes and that the details of individual cases be taken up by wise jurists. On the other hand, it's not a foregone conclusion that all judges are wise or selfless, and of course there might be judges who are both but who would come to quite different conclusions from me because they operate on a different moral calculus. So for the sake of consistency and predictability, there's a strong argument to have laws debated and promulgated by legislators rather than judges, so that anyone can do and look them up for guidance about what is and is not legal. Of course that involves some idealistic assumptions about legislators...

If you like high-density reading material, I strongly recommend How Judges Think by Judge Richard Posner.