Question

1. Why did most halakhic decisors take the verses “and you shall greatly guard your lives / only take heed to yourself and guard your life greatly” out of their context?  After all, those passages deal with the concern lest the children of Israel worship idols, or make images of the deity, after entering the Land. Is this only a scriptural support?
Is the same true of the Talmud in Shevuot 36a, which derives that a person may not curse himself from “take heed to yourself and guard your life”?
2. Why did the Sages enact decrees regarding things connected to danger? Wouldn’t it have been enough simply to warn the public that such-and-such a thing is dangerous?
3. Nadav Shenarav writes (Keren Zavit, Va’etchanan): “In any case, a problem has arisen here that needs attention. The assumption that a person may not expose himself to danger, or the view of ‘and you shall guard yourselves’ as a full-fledged law, led the halakhic decisors (Maimonides in Laws of Murderer chapters 11 and 12, Shulchan Arukh Yoreh De’ah 115) to present a list of things one may not do because of concern for human life: drinking uncovered water, eating fish with meat, ‘one must be careful not to put money in his mouth,’ ‘one should not stick a knife into an etrog or a radish,’ drinking water from rivers at night, and so on and so on.  
There is a problem here. Maimonides and the Shulchan Arukh more or less quoted things stated in the Talmud, on the basis of the scientific knowledge of the Amoraim. In our time, when statistics are widely used to assess the benefit or harm of various health and dietary habits, this effectively creates room to turn anything into Jewish law. Rabbis who go along with this trend have ‘ruled’ that it is forbidden to give parts of the Land of Israel to Arabs, that it is obligatory to give parts of the Land of Israel to Arabs, a prohibition on smoking, on driving above the speed limit, and so on without end. In principle, it is not impossible that some rabbi might decide that travel speed must be limited to 30 km/h (there is no doubt that this would prevent many traffic accidents), or propose drastic restrictions on carbon dioxide emissions for fear of global warming, or forbid consumption of fattening foods—the sky is the limit. 
If we want to prevent the ‘explosion’ of Jewish law, and the cheapening of the rabbinate caused by rabbis becoming the mouthpiece of every health trend and every panic-monger who cloaks his words in a scientific mantle, it seems that we must adopt some principle of common sense of the kind proposed by Rabbi Eliezer Naensikh (comparison with ordinary risks that people take upon themselves, such as sea and desert journeys) and Rabbi Moshe Feinstein (who made the law depend on ‘what people commonly think’).”
 On the one hand I understand the need to prevent the explosion of Jewish law, but on the other hand I don’t understand what the Sages accomplished with their enactment. After all, what is the point of being careful specifically about things the public sees as dangerous when in truth the opposite is the case?