Parshas Va’yakhel begins with the juxtaposition of the commandment to build a Mikdash and the halachos of Shabbos. One of the several things that are learned from this is actually not related specifically to Shabbos. The first pasuk in the parsha says, “Va’ayakel Moshe es kol adas bnei Yisrael vayomer aleihem eileh ha’devarim” – and Moshe gathered all of Bnei Yisrael together and told them the following things. The Baal Haturim on this pasuk quotes the Yalkut Shimoni who explains that we derive from here that Shabbos is a time to gather in the beis midrash and learn Torah together.
The Mishnah in Shabbos (120) says that one is not allowed to read Kesuvim on Shabbos afternoon, since this may lead him to become engrossed in his reading and as a result not attend the shiur in the beis midrash on Shabbos afternoon. The Gemara Megillah 4a says that if Purim falls out on Shabbos we do not read the Megillah because of the gezeira that perhaps someone will come to carry it in rishus harabim. The Yerushalmi (Megillah 3:4) says a different reason for why we do not read the Megillah on Shabbos. It is because of the prohibition not to read Kesuvim on Shabbos day.
This reason is perplexing, for the Gemara in Megillah 3b says that we are mevatel Talmud Torah for the sake of reading the Megillah. The reason why someone is not allowed to read Kesuvim on Shabbos is because perhaps they will not come to the drasha in the beis midrash. In other words, they will not come to learn Torah. This seems to contradict the Gemara that says that we are mevatel Talmud Torah for the sake of reading the Megillah.
I heard from my rebbe, Rav Michel Shurkin, shlita, the following answer: The Rambam was careful where he placed each halacha. Often halachos are derived from which category the Rambam chose to write a certain halacha. Even if a certain halacha appears in the same Gemara as another, the Rambam is known to place it in the section that he felt it belonged. Additionally, the Rambam was particular in grouping certain halachos as well. If two seemingly unrelated halachos were combined into one halacha this indicates that the Rambam believed they were connected. The same applies to the juxtaposition of halachos next to each other
The Rambam writes the halacha that one may not read Kesuvim in hilchos Shabbos (23:19). In that same halacha the Rambam begins with the fact that it is prohibited to read documents on Shabbos, and captions under pictures. In that same halacha the Rambam writes that it is prohibited as well to read Kesuvim on Shabbos afternoon since it may lead one to not attend the shiur in the beis midrash.
The prohibition not to read documents is because of a gezeira that one should not come to write on Shabbos. The basic understanding of why it is prohibited to read Kesuvim on Shabbos afternoon is because it may lead one to skip the drasha in the beis midrash. This is seemingly a halacha in Talmud Torah, and it should have thus been written in hilchos Talmud Torah. Why did the Rambam write this in hilchos Shabbos, and why did he write it together with the halacha not to read documents?
It is apparent from this that the Rambam believed that the prohibition not to read Kesuvim on Shabbos is not part of the halachos of Talmud Torah, rather it is part of hilchos Shabbos. Shabbos requires that one come to the beis midrash to hear a drasha. It is not a halacha that is part of Talmud Torah.
Based on this we can understand why the Yerushalmi that said that the reason one cannot read the Megillah on Shabbos is because of this prohibition not to read Kesuvim on Shabbos is not contradicted by the Gemara that says that we are mevatel Talmud Torah for the sake of reading the Megillah. One can only be mevatel Talmud Torah for the sake of reading the Megillah. However one cannot be mevatel the halachos of Shabbos in order to read the Megillah. As we have established, reading the Megillah is a violation of hilchos Shabbos, therefore one cannot read it on Shabbos.