ושל תרומה טהורה לא יטול: פליגי בה ר' אמי ור' אסי חד אמר מפני שמכשירה וחד אמר מפני שמפסידה
The mishna stated: And with regard to an etrog of pure teruma, one should not take it ab initio. However, if he did, it is fit for use in fulfilling the mitzva. Rabbi Ami and Rabbi Asi disagreed about this. Why may one not take it ab initio? One of them said: It is because one renders it susceptible to impurity. All vegetation cannot become ritually impure, even if it came into contact with a source of impurity, unless it was moistened by one of seven liquids (see Leviticus 11:37–38). However, once one touches the etrog with wet hands, which are wet because he removed the other three species from the water in which they were kept to preserve their freshness, he renders the etrog susceptible to impurity. The Sages prohibited taking an etrog of teruma, lest it become impure, as it is prohibited to impurify teruma. And one said: It is because he damages it. By handling the etrog, the peel is rendered disgusting, and it is prohibited to damage teruma.
According to the reason מפני שמפסידה, why doesn't the Gemara not say that one shouldn't take an Esrog of Shvi-is where there is a din of לאכלה ולא להפסד??
