We previously discussed the issur of lifnei iver, and more specifically the issur of assisting someone in doing and aveira - see here. The dagul m'rvava in his interpretation of the Shach holds that the issur d'rabonon of assisting someone in doing an aveira doesn't apply to one who is intentionally trying to violate (Rav Moshe has a teshuva where he supports the opinion of the dagul me'rvava). Rashi in his commentary on the mishna seems to contradict the assumption of the dagul m'rvava. When the mishna says that one is not allowed to press wine with a Jew who isn't careful about tu'mah because he would be assisting in the violation of causing tu'mah to fruits in E.Y. This doesn't fit in the framework of the D.M. - the Jew who isn't careful with tu'mah is intentionally violating, yet we consider any assistance to be a violation of מסייע ידי עוברי עבירה - supporting one doing an aveira. Furthermore, it seems to be a very different type of assistance in doing an aveira than we typically find. Normally, the assistance is helping prior to the violation thereby enabling the violation (even if he could have done it without assistance), but here the wine is already ta'mei at the time you are assisting him in the pressing. How does that qualify as מסייע ידי עוברי עבירה since the aveira has already been done?
It seems to me that Rashi himself tries to address this issue by writing -
ואסור לסייע ידי עוברי עבירה אלא פורשין מהן כדי שלא ירגיל בכך - Rashi accentuates the concern of encouraging him to continue this practice in the future. Generally, the prohibition of מסייע ידי עוברי עבירה is that you are assisting someone in an aveira that he is about to do. The D.M. holds that this prohibition only applies when he is a person who is unaware of the aveira, but doesn't apply when the person is intentionally trying to do the aveira. The rationale of the D.M. is very simple - there is no violation of lifnei iver because he can do it without me, so the prohibition to be involved emanates from the requirement to prevent Jews from sinning - since he wants to sin and I have no ability to stop him, there is no prohibition in assisting him. This rationale makes sense with the type of מסייע ידי עוברי עבירה that the D.M. is addressing. BUT in our mishna where the prohibition of מסייע ידי עוברי עבירה is not assisting him in the aveira he is about to do, because the aveira is already done - the wine is already ta'mei. The prohibition is not to encourage this practice in the future. Therefore, even if he is intentionally violating, it is prohibited for one to assist him because he is essentially encouraging a practice for the future. The difference between the two cases is that in the D.M. situation there is no harm in assisting him since the aveira would be done anyway, but in our case the harm in assisting him is encouraging a practice that he may be more careful about next time if he realizes that he will have a hard time finding people to help him in his pressing. The type of מסייע ידי עוברי עבירה being addressed in our mishna is more similar to the gemara in gittin 62 which permits telling a goy who is working the land in shemita "your hand should be strengthened". The gemara implies that its permitted to say אחזוקו to a goy since he has not issur to work in shemita, but not to a Jew. This would be a violation of מסייע ידי עוברי עבירה similar to our mishna where you would be encouraging a practice for the future and therefore prohibited to do even if he is intentionally violating.