(condensed from Ein Ayah, Shabbat 1:52 – part I)

Gemara: One should not read by the light of an oil lamp [on Shabbat]. Rava said: If he is an important person, it is permitted. A question was raised from a baraita: "One should not read by the light of an oil lamp, lest he tilt the oil cup. Rabbi Yishmael ben Elisha said: ‘I will read and will not tilt.’ One time, he read and almost tilted. He said: ‘How great are the words of the Scholars, who used to say that one should not read to the light of the candle.’ Rabbi Natan said that [Rabbi Yishmael ben Elisha] read and tilted and wrote [after Shabbat] in his notebook: ‘I read and tilted the oil cup on Shabbat. When the Beit Hamikdash is rebuilt, I will bring a fat sin offering.’"

Ein Ayah: There are two intentions behind Rabbinical prohibitions that distance one from the situation in which he may come to violate a Torah law. The simple intention is to remove the practical stumbling block of a violation. According to this element, the goal of the Rabbinical prohibition is negated only when one ignored the Rabbinical law in a way that actually caused the violation of the Torah law.
However, there is a second intention behind the Rabbinical prohibition. The foundations of fear of Hashem and the fulfillment of the Torah depend on the quality of one’s high view of the loftiness of the Torah law in his heart. When one is personally careful not to put himself into a situation in which there is any concern that he will come to violate a Torah law, the steps that illustrate this concern engrave in his heart a strong imprint of the importance of Torah observance and the damage done by violating even one law of the Torah. This is the psychological result of one who is as careful to stay away from a situation of danger of violating the Torah as to avoid danger to his life or to protect a precious treasure from the remote danger of losing it.
In the inverse, when one places himself into a situation in which he is liable to violate a Torah law, even when in practice he is careful enough to not violate it, his behavior makes an impression in his heart of, in general, lowering the value of fulfilling the Torah. After all, he was not concerned enough to protect himself from a situation that could lead to a Torah violation. This act of laxness about Torah has an impact even if he does end up violating the Torah prohibition.
Therefore, the gemara asks that Rava’s approach that an important person may read next to an oil lamp is contradicted by the source about Rabbi Yishmael ben Elisha, who read and almost tilted the oil cup. We see that even an important person could bring himself to violate Torah law, and therefore even if the person did not sin and there seems to not have been damage, there still was a weakening of the spirit of Torah observance by allowing himself to be close to a violation. That is why Rabbi Yishmael praised the Rabbis for making the Rabbinic prohibition on reading. The Gra (Dvar Eliyahu, p. 67) explains that by not including in the prohibition the reason of not coming to tilt the oil cup, the gemara illustrates that the prohibition is not just to avoid that the cup will be tilted, with the resulting Torah violation, but to avoid the situation (based on the second reason for the Rabbinic prohibition). The question that relates to the first reason behind the prohibition is from Rabbi Natan’s version that he actually did do the Torah violation, as we see that no one can rely on himself not to come to a violation.