In the Torah law, it is written that those people must die. To say that they do not need to be killed is to change that law, or at the very least disregard its instruction. The fact that Jewish courts have disregarded it for centuries is a blessing to all, but they are still ignoring it.
No, we do not ignore it. It lists the maximum penalty for a crime; every crime perpetrated is unique, with a unique criminal, a unique victim (if any), and unique circumstances. All of these must be considered. It
appears to be a harsh, explicit, unbending law upon looking at it, but what exists in the Torah is the codified laws, the statutes of G-d, if you will. These statutes are unbending and unchanging; however, interpretation of the statutes - their intent, the other possible penalties which could be exercised in certain cases, and so on, is evolved within oral traditions, going back even to when the Torah was written.
These oral traditions, the caselaw, if you will, are the interpretations, over several millennia, of the unchanging code of the Torah. Interpretations do, occasionally, change, over time, but passages must be interpreted. The caselaw, the oral traditions, evolve. We have the redacted Mishnah, part of the Talmud, which is the earliest of traditions, which today have a status almost or exactly equal to the original codified Torah. We have the gemara, the other part of the Talmud, which are more recent interpretations of the Mishnah's interpretations of the Torah. Then, there have been rabbis, over millennia since the redaction of the Talmud, which issue rulings, to this day, based upon not only the Torah but on the "caselaw" from it - basing their rulings on the rulings of previous learned scholars in addition to their own studies.
The difference is like that between the Napoleonic Code system and the English Common Law system. In the Napoleonic system, caselaw is technically forbidden. All statutes must be read, as reported, and enforced explicitly as written. At least, in theory, Napoleonic courts act on this. In the English system, however, there is interpretation of statutes, based on older traditions and previous decisions of other courts. For example, in the United States, when a normal judge is making a decision based on a property dispute. He looks to the statutory law established on these bases, statutory law established by the state legislature. But he also looks to caselaw on other property disputes, both in that state and in other places with the same system, sometimes even back centuries in time to English courts, complex disputes sometimes drawing on caselaw going back to the 13th century! This is the most critical. He issues a ruling based on all of these. He does not consider constitutionality; the Constitution itself is not his to interpret! But he bases it on statutes which ultimately derive their authority from the Constitution, and caselaw which derives its authority from the same. Sometimes, caselaw can even override the statutes!
When you look at the Torah, you see a basic law code. You see it and assume that it must be followed directly, because it seems explicit. But it isn't. It is a law code surrounded with traditions through which you must read in order to understand what the passages mean.
