Gittin 60a - Eis La'asos L'hashem

Simply speaking the heter to write down torah sh'bal peh, even though it is a violation of "These you should write", as the gemara says on 60b, is "eis l'asos..." meaning that we violate the Torah in order to preserve it. However, the maharsha points out that the concept of "hefeiru tora'secha", is not just a violation of the Torah but rather a rejection of the whole concept in why we are not allowed to write it down in the first place. The maharsha explains that the reason why Torah Sh'bal Peh must remain Ba'al Peh is that it is impossible to incorporate the entire Torah in writing, therefore the writing of it places a limitation on the Torah which does not exist. Therefore in the earlier generations it was forbidden to write down the oral torah, so that they can use their time more efficiently in reviewing the Torah. However, when people became unable to grasp the entire Torah orally, and began to forget, it was necessary to put it into writing to preserve it. This was not just at the expense of the violation of the prohibition against writing it, but rather, as the maharsha writes: שיהיו המצות סדורים לו בכתיבה אע"פ שעל ידי זה הפרו תורתך, שלא יחזור על משנתו ותלמודו תמיד כי יסמוך על הכתיבה

Meaning the decision to write down the oral Torah was considered destructive to the continuity of Torah, since it would be impossible to fully master. Nevertheless, the decission was made to forfeit the sections that could not be incorporated into writing that would be lost, to preserve the section that could be incorporated into writing that would now be mastered. "Hefeiru Torasecha" wasn't just a violation of an issur, it was to literally push away a section of Torah b'ydayim that would be lost forever. This explains why the pesak was so controversial.