Rav Nachman states that we learn about the issur of squeezing rimonum from the house of Menashya Bar Menachem who would do this during the week. Rava then counters that we cannot learn from this because "is this rov haolam?" Why is this case singled out by Rava? There are numerous instances where we learn halacha k'maeseh of an individual, e.g. especially in bishul with maeseh k'Ben Drosai! Why wasn't this brought down as a retort to Rava rather than going through the "kotzim" argument and then ultimately abandoning this in favor of comparison of Terudin in the Mikveh?
Excellent question. Your question is made even stronger by the words of Tosfos (92b) who says that everyone would squeeze pomegranates for juice if they were as wealthy as the house of Menashya bar Menachem. Certainly this type of squeezing is more fit for all to use than cooking a food item k'Ma'achal Ben Derusai!
It seems that Bishul k'Ma'achal Ben Derusai is considered Bishul not *because* that is how much Ben Derusai would cook his food, but rather that is the *Shi'ur* in the degree of Bishul which the Torah requires in order to be Mechayev someone for cooking on Shabbos. It is a Halachah le'Moshe mi'Sinai, like all of the Shi'urin (Eruvin 4a). In this case, the Halachah l'Moshe mi'Sinai revealed that even a minimal degree of cooking is included in the prohibition of cooking on Shabbos, such as the amount needed to make the food barely edible.
This is not the case with regard to squeezing pomegranates, which has nothing to do with Shiurim. Whether or not one brings a Korban for squeezing pomegranates depends on whether his is *how the world conducts itself*. If it is common to squeeze pomegranates for their juice, one will be Chayav. One will not be Chayav if it is not. In order to determine the Derech of the world, it is certainly necessary to have a large proportion of the world doing this act, or at least everyone in one locality (92b, regarding Anshei Hutzal). This is also the determining factor with regard to Kotzim b'Kerem, which depends on how the people in the world view them.
Thank you, and be well,
-Mordecai