Explanation #1 (Rashi): He comes to answer that all agree that one enters a Sheretz is liable. They argue about removing Tum'ah.

Question: Seemingly, there is no difference! If one who enters a Sheretz is liable, likewise he is commanded to remove it, due to "vi'Yshalchu Min ha'Machaneh." Whenever we apply "Lo Yetam'u Es Mishkani", we apply "vi'Yshalchu";

If so, what is the reason for [Ben Nanas,] who does not obligate removing it from the entire Azarah?

Explanation #2: This is an independent matter. It does not come to reject what was said above, that Tana'im argue about this (whether or not the Mitzvah to expel Tum'ah applies only to what can become Tahor in a Mikveh. R. Yochanan) does not distinguish between entering and removing, for there is no difference, like we explained;

Rather, he comes to give a verse to support the one who says that he is liable, and for the one who exempts.

Explanation: R. Shimon says as follows to the first Tana. Even though I am lenient about one who was outside the Techum when it became dark [on Shabbos night], I am stringent about a harp string, for there (outside the Techum) it is not a leniency, rather, they gave to you what is yours (the Techum is really 15 Amos past where it is marked);

Also here, they gave to you only what is yours, i.e. a bow, which is [totally] permitted.

Implied question: Why did he wait until now [to argue with the first Tana? He should have argued in the Mishnah on 102b. There are four Mishnayos in between!]

Answer: [Rebbi, who codified the Mishnayos,] wanted to teach all of the [Heterim] in the Mikdash, but not outside the Mikdash [before R. Shimon's opinion. We learn that all these Heterim are only for Shevus].