The Mishna discusses the bedika requirements for a shota, chereshes, etc. Rashi says in the Gemara that if she (or he in the case of a baal keri) were to eat teruma betum'as haguf, they'd be over on the lav of umesu vo ki yechaleluhu.
Are we not dealing with someone who is patur from all mitzvos? I can understand if the concern was for the mishmeres teruma, that the teruma not become tamei, regardless of the cause. But that is apparently how Rashi learns?
Gershon
The Sefer Aizehu Mekomon asks exactly the same question, and does not give a reply (besides noting that the Aruch Laner touched on the topic, but doesn't really answer the question). It is possible that Rashi's comment is based on the Igros Moshe's explanation (Y.D. 1:6) that we are not allowed to feed Issur to Shotim because they are generally included in the Mitzvos as part of Bnei Yisrael, and merely have an exemption when they are a Shotah that they do not have to actively keep the Mitzvos at this time (what R' Moshe terms a "Ptur b'Kiyum b'Poel"). Rav Moshe zt"l continues that there is an argument whether one who empowers an Isur to be done by someone who will transgress that Isur b'Ones is guilty of Lifnei Iver mid'Oraisa or mid'Rabanan (Misayei'yah). However, Rav Moshe says, it is definitely forbidden to do so, despite the fact that the person is an Ones. Applying this to our Sugya, Rashi could be emphasizing the Isur for two reasons. Not only is the Shoteh still generally included in the Mitzvos and we should therefore make sure that he does not transgress them in any way, but we should also not empower him to transgress such a serious Isur which is punished under normal circumstances by death, as that would make us guilty of aiding and abetting a sin.
Kol Tuv,
Yaakov Montrose