To Rav Shach when we say R Shimon goes by the reason for the verse that means when both are written, the reason and the isur [[prohibition] then we go by both. So when it says by a king not to have more than 18 wives because she might tilt his heart away from God -- that means we go by both the letter of the law and also the reason.
That is how Rav Shach explains the contradiction in the Rambam where in general he does not go by R Shimon. [Well, no one does because that is R Shimon against the sages. That is the reason the Tur does not decide like R Shimon either.] But in the debate if one can not marry any girl from the nations that worship idols [עכו''ם] or only a girl from the seven Canaanite nations-- it is R. Shimon who says all nations (that worship idols) are forbidden. So why does the Rambam decide the law like that against the sages? Answer (of Rav Shach): because the letter of the law means all nations, and there R Shimon forbids all nations because of the letter of the law.
On the way by from the sea, I thought about this and asked myself, "What about a widow?" There we know R. Shimon says one can take a pledge from a rich widow. And then I realized what Rav Shach means. He means to say that when both the isur (prohibition) and the reason are written, then R Shimon goes by both. But if only one is written, then he goes by the reason.
This is the opposite of R Yehuda. For to R Yehuda, if only the prohibition is written then he goes by the letter of the law,-- but if both are written, then he goes only by the reason for the verse. That is why he say a king can marry as many women as he likes as long as they do not turn his heart.
[This is a three way debate. It is R Shimon who goes by the reason for the verse always. Do we know the reasons for the verses? Yes. No one in the Gemara disagrees with that. If that would be in doubt, then the sages would bring that objection against R Shimon and ask him, "but we do not know the reasons for the verses." And in fact what are the reasons? they are spelled out in Sefer HaHinuch from a disciple of Nahmanides. Generally they are these: peace of the state, to lessen pleasures, to get rid of idolatry, to gain good character.]
