(a) On 15a, the Gemara brings the story of Shammai opening the roof so his newly born grandson could be in the sukka. The Gemara answers that anyone who saw that would assume he only wanted more air in the sukka.
Later, the Gemara brings the din of Shokes Yehu. Here again, the
Gemara answers that anyone who saw that would assume he only wanted more water in the shokes. Here, Rashi comments that we are concerned that it should not look like two batei din in one city .
Why did Rashi wait until here-why did he not make this observation on
the first case of Shammai's grandson? I assumed that the first story was more private, but the Gemara still appears concerned with how it would look and what conclusions people would draw?
(b) Similar type question on Rashi on 16a. When the Gemara says that Amon and Moav give maaser oni, Rashi explains why and says that the Chachomim were concerned that the aniyim should have leket shichcha and peah as well as maaser oni. When the Gemara continues and mentions about kedusha rishona, however, Rashi says that the aniyim will be able to find jobs, and also** get leket shichcha peah and maaser oni. Why did Rashi add this piece about getting jobs only here?
(a) You are raising a very good point. The answer might be that in the case of opening the roof to enlarge the Sukah for the child, when the Gemara answers that "one who sees it will assume that it was done in order to increase the air," the Gemara means that the one who sees will not think that the roof was opened for the sake of the Mitzvah at all, and thus it is obvious that opening the roof will not appear to be following a different Torah. In the case of Shokes Yehu, though, where Beis Shamai attached the water of the Shokes to the Mikvah by making a hole in the majority of the Shokes, it is obvious that there was no other reason why he did it other than to be Machshir the Shokes for Tevilah. Nevertheless, the Gemara answers that one who sees will not think that the large hole was made in order to follow the ruling of Beis Shamai, but will think that it suffices to have a small hole (like a Shefoferes ha'Nod), and the large hole was made simply to make the water mix better, even though there is no benefit in doing so from a Halachic standpoint. Therefore Rashi had to emphasize that even though it is clear that the hole was enlarged in order to be Machshir the Shokes, nevertheless doing so did not create a problem of "Lo Sisgodedu."
(b) This is not a difficulty. At first, Rashi is discussing the words of Rebbi Dosa that "we separate Ma'aser Ani" and not Ma'aser Sheni (as we see clearly in Yadayim 4:3). If the reason was to enable the poor people to hire themselves out during Shevi'is, what difference does it make if we separate Ma'aser Ani or Ma'aser Sheni then?
Later, though, Rashi is discussing the Gemara that says that Kedushah Rishonah was not made permanent in those lands, in order for the poor people to be able to rely on those lands during Shevi'is, as opposed to the view that the Kedushah of those lands was made permanent and all of the laws of Shevi'is are practiced there. Therefore Rashi writes that the reason that the lands were not given Kedushah is because they wanted to give the poor people the ability to hire themselves out to work the land in those places.
Mordecai Kornfeld