פס"ר דלא ניחא ליה

Tosfos in Shabbos 75a and 103a says that a פסיק רישא דלא ניחא ליה is prohibited rabbinically on Shabbos for it is a מלאכה שאין צריך לגופא which is a rabbinic prohibition.  It is clear from Tosfos 103 that he doesn't hold only regarding Shabbos that a פס"ר דלא ניחא ליה is prohibited but by other issurim it is prohibited as well.  The law of מלאכה שאין צריך לגופא is only applicable by Shabbos so it would appear Tosfos must hold by other prohibitions it is forbidden from Torah law.  However, regarding Shabbos one would have been exempt because its not מלאכת מחשבת, nonetheless it's still rabbinically prohibited (see also Tosfos Yoma 35a.)
However, if we look at Tosfos in Ketubot 6a, it appears that Tosfos there has a different take.  There Tofos doesn't say it it a מלאכה שאין צריך לגופא, but that the Rabbis prohibited just like a מלאכה שאין צריך לגופא.  In other words, Tosfos agrees that a פס"ר דלא ניחא ליה itself is not a מלאכה שאין צריך לגופא and technically should be completely permitted.  However, just like the Rabbis prohibited a מלאכה שאין צריך לגופא because it looks like your doing a מלאכה, one can't tell what's going on in your head, so too the Rabbis prohibited a פס"ר דלא ניחא ליה because it looks like you're doing a מלאכה.  According to this Tosfos it may well be that even regarding other issurim the prohibition of פס"ר דלא ניחא ליה is rabbinic in nature.
The Aruch is of the opinion that a פס"ר דלא ניחא ליה is completely permitted.  Tosfos says one of his proofs is from the Gemorah in Zevachim that one may offer a נדבה of wine and pour it on the fire of the alter even though one violates the lav of extinguishing the fire from the mizbaoch for the person's intent is not to extinguish, but to offer his נדבה.  The Aruch asks how is it permitted because its a דבר שאינו מתכוין, its a פס"ר that it will be extinguished?  He says we see that since you don't want the fire to be extinguished, its permitted. Tosfos in Ketubos says no proof; there its permitted to uphold the mitzvah of your libation.  Tosfos in Shabbos answers that the question isn't right; its not a פס"ר, you can pour the wine slowly so it won't extinguish the fire.  Tosfos in Shabbos can't say like Tosfos in Ketubot for saying מצוה שאני only makes sense if its a rabbinic prohibition.  Since Tosfos in Shabbos holds its prohibited from Torah law, he is forced to say its not a פס"ר.
The big question is why according to the Aruch (and even Tosfos in Ketubot that disagrees,) it פס"ר דלא ניחא ליה not prohibited because of מלאכה שאין צריך לגופא?  (See the gilyon on the Tosfos, Rashash and Chidushay Rav Reuvain.)  The reason why a פס"ר דלא ניחא ליה is permitted according to the Aruch is that he holds פס"ר is prohibited for we view it as if the person intended to do the מלאכה since its an obvious derivative of his actions.  However, if its against his will, we can't view it as if the person has intent to do the מלאכה for the individual doesn't want to do the מלאכה at all (see Rosh Shabbos 16:8.)  That is all if there is no intent to do the מלאכה.  However, when there is intent to dd the מלאכה, but the person doesn't desire the results of the מלאכה, then its a מלאכה שאין צריך לגופא (still needs to be worked through all the cases,וצ"ע.)
The Maggid Mishna (Shabbos 12:2) has a chiddush that when it comes to the מלאכה of מכה בפטיש there is no law of פס"ר being prohibited for it is only the though process of the person that determines if what they are working on is completing it to be a vessel.  This seems to fly in the face of the Gemorah Sukkah (33b) that assumes there is a problem of פס"ר when plucking berries off the hadash branch for it makes it into a usable object for the mitzvah?  [It is true the Rambam omits that step of the Gemorah which would work well for this Maggid but the Maggid there (Lulav 8:5) gives a different explanation of the Rambam which still doesn't work with his opinion.]